44 lines
1.3 KiB
Java
44 lines
1.3 KiB
Java
// generics/Holder.java
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// (c)2016 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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import java.util.Objects;
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public class Holder<T> {
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private T value;
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public Holder() {}
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public Holder(T val) { value = val; }
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public void set(T val) { value = val; }
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public T get() { return value; }
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@Override
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public boolean equals(Object o) {
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if (o == this) return true;
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return o instanceof Holder &&
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Objects.equals(value, ((Holder)o).value);
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}
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@Override
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public int hashCode() {
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return Objects.hash(value);
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}
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public static void main(String[] args) {
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Holder<Apple> apple = new Holder<>(new Apple());
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Apple d = apple.get();
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apple.set(d);
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// Holder<Fruit> Fruit = apple; // Cannot upcast
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Holder<? extends Fruit> fruit = apple; // OK
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Fruit p = fruit.get();
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d = (Apple)fruit.get(); // Returns 'Object'
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try {
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Orange c = (Orange)fruit.get(); // No warning
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} catch(Exception e) { System.out.println(e); }
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// fruit.set(new Apple()); // Cannot call set()
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// fruit.set(new Fruit()); // Cannot call set()
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System.out.println(fruit.equals(d)); // OK
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}
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}
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/* Output:
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java.lang.ClassCastException: Apple cannot be cast to
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Orange
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false
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*/
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