有这样一个面试题:
String a = "a"; String a1 = new String("a"); String a2 = a1.trim() + ""; String a3 = "a" + ""; String a4 = "a".trim() + ""; System.out.println(a == a1); System.out.println(a.intern() == a1.intern()); System.out.println(a2 == a1); System.out.println(a3 == a); System.out.println(a4 == a);
请问结果是什么?
运行后的结果是:
false true false true false
为什么结果是这样呢?这需要从String 本身说起。
Java在运行时会维护一个字符串常量池String Pool; 在String a="a"; 首先检查字符串常量池中是否有"a",如果有则直接返回,否则在常量池中创建一个新的;
String a1=new String("a"); 使用new 关键字创建的对象一定在堆栈中,同样也会维护字符串常量池,因为字符串常量池中已经存在了,则不会添加新的。在JAVA中==永远都是比较两个内存地址是否相同,这样因为a和a1不是同一个对象则已定返回false;
intern()方法时返回字符串常量池中的对象,因为常量池中只存在一个“a” 则两者已定相等;
a2.trim()+"" 则是在堆栈中创建了一个新对象,同时维护常量池;则该表达式返回false 但是字符串常量的拼接仅仅维护常量池不会在堆栈中创建新对象则"a"+""还是常量池中的“a”;
为了更直观了解我们可以看一下这段代码的字节码,通过字节码可能更有说服力。
LDC是将常量推送到栈顶,ASTORE 是将栈顶元素赋值给本地变量。
iinvokespecial 调用实例初始化方法,父类实例化方法,私有方法
iinvokevirtual虚方法
通过字节码可以发现 a是常量指向的地址是#2,a1是通过常量“a"new出来的地址是,a2也是new出来的,a3还是常量“a" #2,a4也是new出来的
public class core.demo.StringTest { public core.demo.StringTest(); 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 a 2: astore_1 3: new #3 // class java/lang/String 6: dup 7: ldc #2 // String a 9: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V 12: astore_2 13: new #5 // class java/lang/StringBuilder 16: dup 17: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V 20: aload_2 21: invokevirtual #7 // Method java/lang/String.trim:()Ljava/lang/String; 24: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 27: ldc #9 // String 29: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 32: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 35: astore_3 36: ldc #2 // String a 38: astore 4 40: new #5 // class java/lang/StringBuilder 43: dup 44: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V 47: ldc #2 // String a 49: invokevirtual #7 // Method java/lang/String.trim:()Ljava/lang/String; 52: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 55: ldc #9 // String 57: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 60: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 63: astore 5 65: getstatic #11 // Field java/lang/System.out:Ljava/io/PrintStream; 68: aload_1 69: aload_2 70: if_acmpne 77 73: iconst_1 74: goto 78 77: iconst_0 78: invokevirtual #12 // Method java/io/PrintStream.println:(Z)V 81: getstatic #11 // Field java/lang/System.out:Ljava/io/PrintStream; 84: aload_1 85: invokevirtual #13 // Method java/lang/String.intern:()Ljava/lang/String; 88: aload_2 89: invokevirtual #13 // Method java/lang/String.intern:()Ljava/lang/String; 92: if_acmpne 99 95: iconst_1 96: goto 100 99: iconst_0 100: invokevirtual #12 // Method java/io/PrintStream.println:(Z)V 103: getstatic #11 // Field java/lang/System.out:Ljava/io/PrintStream; 106: aload_3 107: aload_2 108: if_acmpne 115 111: iconst_1 112: goto 116 115: iconst_0 116: invokevirtual #12 // Method java/io/PrintStream.println:(Z)V 119: getstatic #11 // Field java/lang/System.out:Ljava/io/PrintStream; 122: aload 4 124: aload_1 125: if_acmpne 132 128: iconst_1 129: goto 133 132: iconst_0 133: invokevirtual #12 // Method java/io/PrintStream.println:(Z)V 136: getstatic #11 // Field java/lang/System.out:Ljava/io/PrintStream; 139: aload 5 141: aload_1 142: if_acmpne 149 145: iconst_1 146: goto 150 149: iconst_0 150: invokevirtual #12 // Method java/io/PrintStream.println:(Z)V 153: return }