121 lines
3.9 KiB
Java
121 lines
3.9 KiB
Java
// generics/Wildcards.java
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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); // Warning:
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// Unchecked call to set(T) as a
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// member of the raw type Holder
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// holder.set(new Wildcards()); // Same 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); // Error:
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// set(capture of ?) in Holder<capture of ?>
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// cannot be applied to (Object)
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// holder.set(new Wildcards()); // Same 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); // Error:
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// set(capture of ? extends T) in
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// Holder<capture of ? extends T>
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// cannot be applied to (T)
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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(); // Error:
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// Incompatible types: found Object, required T
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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); // Warnings:
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// Unchecked conversion from Holder to Holder<T>
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// Unchecked method invocation: exact1(Holder<T>)
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// is applied to (Holder)
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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); // Warnings:
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// Unchecked conversion from Holder to Holder<Long>
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// Unchecked method invocation: exact2(Holder<T>,T)
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// is applied to (Holder,Long)
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Long r6 = exact2(qualified, lng);
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// Long r7 = exact2(unbounded, lng); // Error:
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// exact2(Holder<T>,T) cannot be applied to
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// (Holder<capture of ?>,Long)
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// Long r8 = exact2(bounded, lng); // Error:
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// exact2(Holder<T>,T) cannot be applied
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// to (Holder<capture of ? extends Long>,Long)
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// Long r9 = wildSubtype(raw, lng); // Warnings:
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// Unchecked conversion from Holder
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// to Holder<? extends Long>
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// Unchecked method invocation:
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// wildSubtype(Holder<? extends T>,T) is
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// applied to (Holder,Long)
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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); // Warnings:
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// Unchecked conversion from Holder
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// to Holder<? super Long>
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// Unchecked method invocation:
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// wildSupertype(Holder<? super T>,T)
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// is applied to (Holder,Long)
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wildSupertype(qualified, lng);
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// wildSupertype(unbounded, lng); // Error:
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// wildSupertype(Holder<? super T>,T) cannot be
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// applied to (Holder<capture of ?>,Long)
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// wildSupertype(bounded, lng); // Error:
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// wildSupertype(Holder<? super T>,T) cannot be
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// applied to (Holder<capture of ? extends Long>,Long)
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}
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}
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/* Output: (None) */
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