284 lines
9.3 KiB
Java
284 lines
9.3 KiB
Java
// generics/Wildcards.java
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// (c)2017 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://OnJava8.com for more book information.
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// Exploring the meaning of wildcards
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public class Wildcards {
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// Raw argument:
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static void rawArgs(Holder holder, Object arg) {
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//- holder.set(arg);
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// warning: [unchecked] unchecked call to set(T)
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// as a member of the raw type Holder
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// holder.set(arg);
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// ^
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// where T is a type-variable:
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// T extends Object declared in class Holder
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// 1 warning
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// Can't do this; don't have any 'T':
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// T t = holder.get();
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// OK, but type information is lost:
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Object obj = holder.get();
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}
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// Similar to rawArgs(), but errors instead of warnings:
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static void unboundedArg(Holder<?> holder, Object arg) {
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//- holder.set(arg);
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// error: method set in class Holder<T>
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// cannot be applied to given types;
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// holder.set(arg);
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// ^
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// required: CAP#1
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// found: Object
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// reason: argument mismatch;
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// Object cannot be converted to CAP#1
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// where T is a type-variable:
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// T extends Object declared in class Holder
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Object from capture of ?
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// 1 error
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// Can't do this; don't have any 'T':
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// T t = holder.get();
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// OK, but type information is lost:
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Object obj = holder.get();
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}
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static <T> T exact1(Holder<T> holder) {
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return holder.get();
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}
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static <T> T exact2(Holder<T> holder, T arg) {
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holder.set(arg);
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return holder.get();
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}
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static <T>
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T wildSubtype(Holder<? extends T> holder, T arg) {
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//- holder.set(arg);
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// error: method set in class Holder<T#2>
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// cannot be applied to given types;
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// holder.set(arg);
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// ^
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// required: CAP#1
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// found: T#1
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// reason: argument mismatch;
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// T#1 cannot be converted to CAP#1
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// where T#1,T#2 are type-variables:
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// T#1 extends Object declared in method
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// <T#1>wildSubtype(Holder<? extends T#1>,T#1)
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// T#2 extends Object declared in class Holder
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends T#1 from capture of ? extends T#1
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// 1 error
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return holder.get();
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}
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static <T>
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void wildSupertype(Holder<? super T> holder, T arg) {
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holder.set(arg);
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//- T t = holder.get();
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// error: incompatible types:
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// CAP#1 cannot be converted to T
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// T t = holder.get();
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// ^
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// where T is a type-variable:
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// T extends Object declared in method
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// <T>wildSupertype(Holder<? super T>,T)
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Object super:
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// T from capture of ? super T
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// 1 error
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// OK, but type information is lost:
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Object obj = holder.get();
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}
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public static void main(String[] args) {
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Holder raw = new Holder<>();
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// Or:
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raw = new Holder();
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Holder<Long> qualified = new Holder<>();
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Holder<?> unbounded = new Holder<>();
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Holder<? extends Long> bounded = new Holder<>();
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Long lng = 1L;
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rawArgs(raw, lng);
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rawArgs(qualified, lng);
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rawArgs(unbounded, lng);
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rawArgs(bounded, lng);
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unboundedArg(raw, lng);
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unboundedArg(qualified, lng);
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unboundedArg(unbounded, lng);
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unboundedArg(bounded, lng);
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//- Object r1 = exact1(raw);
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// warning: [unchecked] unchecked method invocation:
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// method exact1 in class Wildcards is applied
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// to given types
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// Object r1 = exact1(raw);
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// ^
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// required: Holder<T>
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// found: Holder
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact1(Holder<T>)
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// warning: [unchecked] unchecked conversion
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// Object r1 = exact1(raw);
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// ^
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// required: Holder<T>
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// found: Holder
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact1(Holder<T>)
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// 2 warnings
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Long r2 = exact1(qualified);
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Object r3 = exact1(unbounded); // Must return Object
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Long r4 = exact1(bounded);
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//- Long r5 = exact2(raw, lng);
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// warning: [unchecked] unchecked method invocation:
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// method exact2 in class Wildcards is
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// applied to given types
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// Long r5 = exact2(raw, lng);
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// ^
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// required: Holder<T>,T
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// found: Holder,Long
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact2(Holder<T>,T)
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// warning: [unchecked] unchecked conversion
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// Long r5 = exact2(raw, lng);
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// ^
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// required: Holder<T>
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// found: Holder
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact2(Holder<T>,T)
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// 2 warnings
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Long r6 = exact2(qualified, lng);
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//- Long r7 = exact2(unbounded, lng);
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// error: method exact2 in class Wildcards
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// cannot be applied to given types;
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// Long r7 = exact2(unbounded, lng);
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// ^
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// required: Holder<T>,T
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// found: Holder<CAP#1>,Long
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// reason: inference variable T has
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// incompatible bounds
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// equality constraints: CAP#1
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// lower bounds: Long
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact2(Holder<T>,T)
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Object from capture of ?
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// 1 error
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//- Long r8 = exact2(bounded, lng);
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// error: method exact2 in class Wildcards
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// cannot be applied to given types;
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// Long r8 = exact2(bounded, lng);
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// ^
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// required: Holder<T>,T
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// found: Holder<CAP#1>,Long
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// reason: inference variable T
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// has incompatible bounds
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// equality constraints: CAP#1
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// lower bounds: Long
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>exact2(Holder<T>,T)
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Long from
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// capture of ? extends Long
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// 1 error
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//- Long r9 = wildSubtype(raw, lng);
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// warning: [unchecked] unchecked method invocation:
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// method wildSubtype in class Wildcards
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// is applied to given types
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// Long r9 = wildSubtype(raw, lng);
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// ^
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// required: Holder<? extends T>,T
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// found: Holder,Long
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSubtype(Holder<? extends T>,T)
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// warning: [unchecked] unchecked conversion
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// Long r9 = wildSubtype(raw, lng);
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// ^
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// required: Holder<? extends T>
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// found: Holder
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSubtype(Holder<? extends T>,T)
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// 2 warnings
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Long r10 = wildSubtype(qualified, lng);
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// OK, but can only return Object:
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Object r11 = wildSubtype(unbounded, lng);
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Long r12 = wildSubtype(bounded, lng);
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//- wildSupertype(raw, lng);
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// warning: [unchecked] unchecked method invocation:
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// method wildSupertype in class Wildcards
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// is applied to given types
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// wildSupertype(raw, lng);
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// ^
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// required: Holder<? super T>,T
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// found: Holder,Long
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSupertype(Holder<? super T>,T)
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// warning: [unchecked] unchecked conversion
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// wildSupertype(raw, lng);
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// ^
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// required: Holder<? super T>
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// found: Holder
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSupertype(Holder<? super T>,T)
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// 2 warnings
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wildSupertype(qualified, lng);
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//- wildSupertype(unbounded, lng);
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// error: method wildSupertype in class Wildcards
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// cannot be applied to given types;
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// wildSupertype(unbounded, lng);
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// ^
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// required: Holder<? super T>,T
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// found: Holder<CAP#1>,Long
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// reason: cannot infer type-variable(s) T
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// (argument mismatch; Holder<CAP#1>
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// cannot be converted to Holder<? super T>)
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSupertype(Holder<? super T>,T)
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Object from capture of ?
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// 1 error
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//- wildSupertype(bounded, lng);
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// error: method wildSupertype in class Wildcards
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// cannot be applied to given types;
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// wildSupertype(bounded, lng);
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// ^
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// required: Holder<? super T>,T
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// found: Holder<CAP#1>,Long
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// reason: cannot infer type-variable(s) T
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// (argument mismatch; Holder<CAP#1>
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// cannot be converted to Holder<? super T>)
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// where T is a type-variable:
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// T extends Object declared in
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// method <T>wildSupertype(Holder<? super T>,T)
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// where CAP#1 is a fresh type-variable:
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// CAP#1 extends Long from capture of
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// ? extends Long
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// 1 error
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}
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}
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