Java之HashMap get方法__Java

HashMap 非安全執行緒

通過下圖可以看出HashMap的get方法的實現，其判斷標準是通過hashCode和equals方法。

故可以通過重寫類的hashCode和equals方法實現自訂Object的相等標準，完整代碼如下

Book.java

package com.shc.map;public class Book {private String title;public Book(String title) {super();this.title = title;}public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}@Overridepublic int hashCode() {return title.hashCode();}@Overridepublic boolean equals(Object obj) {boolean flag = false;if(obj instanceof Book){Book book = (Book)obj;flag = this.title.equals(book.title);}return flag;}}

HashMapTest.java

package com.shc.map;import java.util.HashMap;import java.util.Map;import org.junit.Test;public class HashMapTest {@Testpublic void testHashMap1(){Map map = new HashMap();String sss = "aaa";String yyy = "bbb";Book aaa = new Book(sss);Book bbb = new Book(yyy);map.put(aaa, "aaaaaaaaaaaaaaaaaa");map.put(bbb, "bbbbbbbbbbbbbbbbbb");Object content = map.get(aaa);//輸出aaaaaaaaaaaaaaaaaaSystem.out.println(content);//改變對象的屬性，其hashCode不變，對象記憶體位址不變aaa.setTitle("xwegwear");content = map.get(aaa);//輸出aaaaaaaaaaaaaaaaaaSystem.out.println(content);//hashCode不同Book ccc = new Book(yyy);content = map.get(ccc);//輸出nullSystem.out.println(content);}/** * 重寫了book的hashCode和equals方法之後 */@Testpublic void testHashMap2(){Map map = new HashMap();String sss = "aaa";String yyy = "bbb";Book aaa = new Book(sss);Book bbb = new Book(yyy);map.put(aaa, "aaaaaaaaaaaaaaaaaa");map.put(bbb, "bbbbbbbbbbbbbbbbbb");Object content = map.get(aaa);//輸出aaaaaaaaaaaaaaaaaaSystem.out.println(content);//改變對象的屬性aaa.setTitle("xwegwear");//map.get時，擷取到的aaa的hashCode已經改變，已經取不到aaacontent = map.get(aaa);//輸出nullSystem.out.println(content);//hashCode相同Book ccc = new Book(yyy);content = map.get(ccc);//輸出bbbbbbbbbbbbbbbbbbSystem.out.println(content);//字串"bbb" 和 變數yyy 的hashCode相同Book ttt = new Book("bbb");content = map.get(ttt);//輸出bbbbbbbbbbbbbbbbbbSystem.out.println(content);}}