Java面试题之 String

        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
}