[Java] equals 與 ==

今天被同事問到Java equals的問題，發現觀念已經有點模糊了…老啦

只記得數字就用==，字串就用equals，至於原因已經還給老師了

Sample:

一開始真的有點打結，不同的物件怎麼會有相同的hashCode…是我的記憶有了錯誤嗎XD

想不通只好各種解釋…

String s1 = new String("ABC");
String s2 = new String("ABC");
System.out.println("s1." + s1 + ".hashCode:" + s1.hashCode());
System.out.println("s2." + s2 + ".hashCode:" + s2.hashCode());

if(s1.equals(s2)){
 System.out.println("s1 is equals s2 !");
}else{
 System.out.println("s1 is not equals s2 !");
}

if(s1 == s2){
 System.out.println("s1 = s2 !");
}else{
 System.out.println("s1 != s2 !");
}

Object o1 = new Object();
Object o2 = new Object();
System.out.println("o1." + o1 + ".hashCode:" + o1.hashCode());
System.out.println("o2." + o2 + ".hashCode:" + o2.hashCode());

if(o1.equals(o2)){
 System.out.println("o1 is equals o2 !");
}else{
 System.out.println("o1 is not equals o2 !");
}

if(o1 == o2){
 System.out.println("o1 = o2 !");
}else{
 System.out.println("o1 != o2 !");
}

Console log:

s1.ABC.hashCode:64578
s2.ABC.hashCode:64578
s1 is equals s2 !
s1 != s2 !
o1.java.lang.Object@7852e922.hashCode:2018699554
o2.java.lang.Object@4e25154f.hashCode:1311053135
o1 is not equals o2 !
o1 != o2 !

後來才想到可以看一下Source Code

原來String有去覆寫hashCode()及equals()

hashCode()改成只要相同valeu就會得到相同的hashCode

equals()也是改成比較value是否相同

而原Object的hashCode()應該是每個不同的物件都會有不同的hashCode

原Object的equals()則是直接回傳==的結果~

String source code:

private int hash; // Default to 0

public int hashCode() {
    int h = hash;
    if (h == 0 && value.length > 0) {
        char val[] = value;

        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
    }
    return h;
}

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
    }
    return false;
}