先貼一段代碼
public static void main(String[] args){
String s1 = "abc";
String s2 = "a";
String s3 = "bc";
String s4 = "a" + "bc";
String s5 = "a" + s3;
String s6 = s2 + s3;
System.out.println(s1 == s4); //true
System.out.println(s1 == s5); //false
System.out.println(s1 == s6); //false
}
程序輸出最后是true,false,false芜赌。之前一直不太懂為什么是這個輸出結果，這幾天在學字節(jié)碼述么，于是查看了這段程序的字節(jié)碼而昨。
public class com.neo.test.Test {
public com.neo.test.Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2 // String abc
2: astore_1
3: ldc #3 // String a
5: astore_2
6: ldc #4 // String bc
8: astore_3
9: ldc #2 // String abc
11: astore 4
13: new #5 // class java/lang/StringBuilder
16: dup
17: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
20: ldc #3 // String a
22: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
25: aload_3
26: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
29: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
32: astore 5
34: new #5 // class java/lang/StringBuilder
37: dup
38: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
41: aload_2
42: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
45: aload_3
46: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
49: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
52: astore 6
54: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
57: aload_1
58: aload 4
60: if_acmpne 67
63: iconst_1
64: goto 68
67: iconst_0
68: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
71: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
74: aload_1
75: aload 5
77: if_acmpne 84
80: iconst_1
81: goto 85
84: iconst_0
85: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
88: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
91: aload_1
92: aload 6
94: if_acmpne 101
97: iconst_1
98: goto 102
101: iconst_0
102: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
105: return
}
從字節(jié)碼中可以看出来氧，字符串"+"其實是調用的StringBuilder的append方法太示，在每次執(zhí)行"+"的時候都會先new一個StringBuilder對象來存儲需要相加的字符串侥锦，最后通過toString()方法來轉換成String挽荠。因此得到的String是一個新的對象克胳。所以s1和s5,s6的地址是不同的。
在代碼中s4 = "a" +"bc"對應的字節(jié)碼是9: ldc #2 // String abc
圈匆，和第一句s1 = "abc"對應的字節(jié)碼是一樣的漠另，在網上查找之后，在字符串常量相加時候跃赚，預編譯時候會把"+"優(yōu)化笆搓，直接把兩個字符串常量自動合成一個字符串常量性湿，因此s1和s4其實是常量池中同一個字符串"abc"的引用，因此返回結果是true满败。
這僅僅是學習記錄肤频。。算墨∠模可能有表達出錯或者我理解出錯的地方，如果有大佬看到了本咸魚的筆記emmmm歡迎指正～
