Question

String a = "abc";
String b = "xyz";
String result = a + b;

我想知道“result”字符串是字符串池中的String常量分配内存还是堆上创建的新对象。

我知道新的String（）在堆上创建对象，并且在permgen字符串池空间中的上例中的字符串常量如a，b。

Answer 1

重要提示：

String a = "abc";
String b = "xyz";
String result = a + b;

与

相同

// creates a number of objects.
String result = new StringBuilder().append(a).append(b).toString();

但

final String a = "abc";
final String b = "xyz";
String result = a + b;

与

相同

String result = "abcxyz"; // creates no new objects.

Answer 2

如果您编译并反编译代码，它将给出以下结果：

String result = new StringBuilder().append(a).append(b).toString();

Answer 3

连接导致分配StringBuilder以创建连接字符串。

来源：

public class Hello {

    public static final String CONST1 = "cafe";
    public static final String CONST2 = "babe";

    public static void main(String[] args){
        String a = "abc";
        String b = "xyz";
        String result = a + b;

        String result2 = CONST1 + CONST2;
    }
}

通过javap反汇编：

public class Hello extends java.lang.Object{
public static final java.lang.String CONST1;

public static final java.lang.String CONST2;

public Hello();
  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 xyz
   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 cafebabe
   27:  astore  4
   29:  return

}

您可以在第10行看到StringBuilder分配，用于连接字符串a和b。请注意，编译器在第25行处理CONST1和CONST2的串联。因此，如果您的字符串为final，则不会导致StringBuilder分配

使用+运算符分配字符串内存

3 个答案: