String,StringBuilder和StringBuffer的区别 - 每天5分钟搞定Java面试

Java平台提供了两种类型的字符串：String和StringBuffer/StringBuilder，它们可以储存和操作字符串。其中String是只读字符串，也就意味着String引用的字符串内容是不能被改变的。而StringBuffer/StringBuilder类表示的字符串对象可以直接进行修改。StringBuilder是Java 5中引入的，它和StringBuffer的方法完全相同，区别在于它是在单线程环境下使用的，因为它的所有方面都没有被synchronized修饰，因此它的效率也比StringBuffer要高。

String对象的intern方法会得到字符串对象在常量池中对应的版本的引用（如果常量池中有一个字符串与String对象的equals结果是true），如果常量池中没有对应的字符串，则该字符串将被添加到常量池中，然后返回常量池中字符串的引用；

看下面这段代码

/**
 * Created by lujie on 2018/9/5.
 */
public class StringEqualTest {
    public static void main(String[] args) {
        String s1 = "Programming";
        String s2 = new String("Programming");
        String s3 = "Program";
        String s4 = "ming";
        String s5 = "Program" + "ming";
        String s6 = s3 + s4;
        System.out.println(s1 == s2); //false
        System.out.println(s1 == s5); //true
        System.out.println(s1 == s6); //false
        System.out.println(s1 == s6.intern()); //true
        System.out.println(s2 == s2.intern()); //false
    }
}

字符串的+操作其本质是创建了StringBuilder对象进行append操作，然后将拼接后的StringBuilder对象用toString方法处理成String对象。
通过查看字节码可以得知。

StringTest.java

/**
 * Created by lujie on 2018/9/5.
 */
public class StringTest {
    public static void main(String[] args) {
        String a = "Hello";
        String b = "World";
        String c = a + b ;
        String d = "Hello" + "World";
    }
}

 public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=5, args_size=1
         0: ldc           #2                  // String Hello
         2: astore_1
         3: ldc           #3                  // String World
         5: astore_2
         6: new           #4                  // class java/lang/StringBuilder
         9: dup
        10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
        13: aload_1
        14: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        17: aload_2
        18: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        21: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        24: astore_3
        25: ldc           #8                  // String HelloWorld
        27: astore        4
        29: return
      LineNumberTable:
        line 8: 0
        line 9: 3
        line 10: 6
        line 11: 25
        line 12: 29
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      30     0  args   [Ljava/lang/String;
            3      27     1     a   Ljava/lang/String;
            6      24     2     b   Ljava/lang/String;
           25       5     3     c   Ljava/lang/String;
           29       1     4     d   Ljava/lang/String;
}