在java中如何连接字符串，"+"起到什么作用

相关学习推荐：java基础教程

下面有段简单的代码

public class StringPlusTest{

    public static void main(String[] args) {
    
        String s1 = "aaa";
    
        String s2 = "bbb";
    
        String s = "ccc" +s1 + s2 +"ddd";

    }
}

使用 javap -c .\StringPlusTest.class反编译一下，得到

Compiled from "StringPlusTest.java"public class com.epoint.codetuning.test.StringPlusTest {
    
  public com.epoint.codetuning.test.StringPlusTest();
    
    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 aaa
       2: astore_1       3: ldc           #3                  // String bbb
       5: astore_2       6: new           #4                  // class java/lang/StringBuilder
       9: dup      10: invokespecial #5                  // Method java/lang/StringBuilder."init>
    ":()V
      13: ldc           #6                  // String ccc
      15: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;
    )Ljava/lang/StringBuilder;
    
      18: aload_1      19: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;
    )Ljava/lang/StringBuilder;
    
      22: aload_2      23: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;
    )Ljava/lang/StringBuilder;
    
      26: ldc           #8                  // String ddd
      28: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;
    )Ljava/lang/StringBuilder;
    
      31: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

      34: astore_3      35: return}

其中，

ldc 把常量池中的项压入栈
astore_1 将引用类型或returnAddress类型值存入局部变量1
astore_2 将引用类型或returnAddress类型值存入局部变量2
new 创建一个新对象
dup 复制栈顶部一个字长内容
invokespecial 根据编译时类型来调用实例方法
aload_1 从局部变量1中装载引用类型值
aload_2 从局部变量2中装载引用类型值
astore_3 将引用类型或returnAddress类型值存入局部变量3

更多指令见JVM指令手册

对于Java来说，这段代码原理上应该是：

public class StringPlusTest{

    public static void main(String[] args) {
    
        String s1 = "aaa";
    
        String s2 = "bbb";
    
        String s = new StringBuilder().append("ccc").append(s1).append(s2).append("ddd").toString();

    }
}

由此可见，Java中使用“+”拼接字符串的实现原理是通过建立临时StringBuilder对象调用append和toString方法实现。

对上面代码做一些修改

public class StringPlusTest{

    public static void main(String[] args) {
    
        String s1 = "aaa";
    
        String s2 = null;
    
        String s = "ccc" +s1 + s2 +"ddd";
    
        System.out.println(s);

    }
}

结果是什么？

运行可以得到如下：

cccaaanullddd

查看StringBuilder源码

@Overridepublic StringBuilder append(String str) {
    
    super.append(str);
    
    return this;
}

该方法调用继承父类AbstractStringBuilder的方法，再去父类中查看

 public AbstractStringBuilder append(String str) {
    
        if (str == null)
            return appendNull();
    
        int len = str.length();
    
        ensureCapacityInternal(count + len);
    
        str.getChars(0, len, value, count);
    
        count += len;
    
        return this;
}
private AbstractStringBuilder appendNull() {
    
    int c = count;
    
    ensureCapacityInternal(c + 4);
    
    final char[] value = this.value;
    
    value[c++] = 'n';
    
    value[c++] = 'u';
    
    value[c++] = 'l';
    
    value[c++] = 'l';
    
    count = c;
    
    return this;
}
    

appendNull方法简单来说就是容量+4，追加null字符串。

所以结果为”cccaaanullddd”。