package hashMap;import java.util.HashMap; import java.util.Map;/* * 可變對象是指創建后自身狀態能改變的對象。換句話說，可變對象是該對象在創建后它的哈希值可能被改變。在下面的代碼中，對象MutableKey的鍵在創建時變量 i=10 j=20，哈希值是1291。然后我們改變實例的變量值，該對象的鍵 i 和 j 從10和20分別改變成30和40。現在Key的哈希值已經變成1931。顯然，這個對象的鍵在創建后發生了改變。所以類MutableKey是可變的。*/ class MutableKey {private int i;private int j;public MutableKey(int i, int j) {this.i = i;this.j = j;}public final int getI() {return i;}public final void setI(int i) {this.i = i;}public final int getJ() {return j;}public final void setJ(int j) {this.j = j;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + i;result = prime * result + j;return result;}@Overridepublic boolean equals(Object obj) {if (this == obj) {return true;}if (obj == null) {return false;}if (!(obj instanceof MutableKey)) {return false;}MutableKey other = (MutableKey) obj;if (i != other.i) {return false;}if (j != other.j) {return false;}return true;} }public class objectAsKey {private static void simulateObjectLost() {// HashMapMap<MutableKey, String> map = new HashMap<>();// Object createdMutableKey key = new MutableKey(10, 20);// Insert entry.map.put(key, "Robin");// This line will print 'Robin'System.out.println(map.get(key));// Object State is changed after object creation.// i.e. Object hash code will be changed.key.setI(30);// This line will print null as Map would be unable to retrieve the entry.System.out.println(map.get(key));}public static void main(String[] args) {// Object createdMutableKey key = new MutableKey(10, 20);System.out.println("Hash code: " + key.hashCode());// Object State is changed after object creation.key.setI(30);key.setJ(40);System.out.println("Hash code: " + key.hashCode());simulateObjectLost();} }

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