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Java面试题之最扯淡的String

2024-09-22 03:28:46   203人阅读


SB里面的toString方法如下,为什么需要这个,查看JVM虚拟机指令用,+号会变成new SB()的,然后调用toString方法
  public String toString() {        // Create a copy, don‘t share the array        return new String(value, 0, count);    }

　　


public class ThreadException{	public static void main(String[] args)	{
　　　　　　　　没加final的代码		String hello = "hello";		String hel = "hel";		String lo = "lo";		System.out.println(hello == "hel" + "lo");		System.out.println(hello == "hel" + lo);	}}
没加final的代码反编译回来后的代码
import java.io.PrintStream;public class ThreadException{public static void main(String[] paramArrayOfString){String str1 = "hello";String str2 = "hel";String str3 = "lo";System.out.println(str1 == "hello");System.out.println(str1 == "hel" + str3);}}
没加final的虚拟机执行的指令集如下public static void main(java.lang.String[]);Code:0: ldc #2 // String hello2: astore_13: ldc #3 // String hel5: astore_26: ldc #4 // String lo8: astore_3   //将操作数栈顶的值保存在本地变量表,变量地址放本地变量表3位置  9: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;12: aload_113: ldc #2 // String hello15: if_acmpne 2218: iconst_119: goto 2322: iconst_023: invokevirtual #6 // Method java/io/PrintStream.println:(Z)V26: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;29: aload_130: new #7 // class java/lang/StringBuilder33: dup34: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V37: ldc #3 // String hel　　　　//这句加载常量到操作数栈  hel39: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;42: aload_3　　　　　　　　　　//加载一个本地变量到操作数栈,从本地变量表3这个位置,将里面的值放到操作数栈上去43: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;46: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;//SB.tostring49: if_acmpne 5652: iconst_153: goto 5756: iconst_057: invokevirtual #6 // Method java/io/PrintStream.println:(Z)V60: return

　　再看下面这个,多加了一个final

加了final的代码
public class ThreadException{	public static void main(String[] args)	{		String hello = "hello";		String hel = "hel";		final String lo = "lo";		System.out.println(hello == "hel" + "lo");//true		System.out.println(hello == "hel" + lo);//true	}}
加了final后反编译回来的代码
import java.io.PrintStream;public class ThreadException{  public static void main(String[] paramArrayOfString)  {    String str4 = "hello";    String str5 = "hel";    System.out.println(str4 == "hello");    System.out.println(str4 == "hello");  }}

看下加了final的虚拟机代码 public static void main(java.lang.String[]);Code:0: ldc #2 // String hello   //加载常量到操作数栈,常量是hello2: astore_13: ldc #3 // String hel5: astore_26: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;9: aload_110: ldc #2 // String hello12: if_acmpne 1915: iconst_116: goto 2019: iconst_020: invokevirtual #5 // Method java/io/PrintStream.println:(Z)V23: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;26: aload_127: ldc #2 // String hello  加载常量29: if_acmpne 36　　　　比较32: iconst_133: goto 3736: iconst_037: invokevirtual #5 // Method java/io/PrintStream.println:(Z)V40: return}

　　再来看这段代码

public class ThreadException{
　　加了final	public static void main(String[] args)	{		String s = new String("abc");		final String s1 = "abc";		String s2 = new String("abc");		final String s3 = "abc";		System.out.println(s == s.intern());		System.out.println(s1 == s2.intern());		System.out.println(s2.intern() == s2.intern());		System.out.println(s3 == s1);	}}
加了final反编译回来的代码import java.io.PrintStream;public class ThreadException{  public static void main(String[] paramArrayOfString)  {    String str1 = new String("abc");    String str2 = new String("abc");    System.out.println(str1 == str1.intern());    System.out.println("abc" == str2.intern());    System.out.println(str2.intern() == str2.intern());//这段是不是很惊讶    System.out.println(true);  }}是不是很扯淡?

　　再看没加final的

public class ThreadException{	public static void main(String[] args)	{　　没加final的代码		String s = new String("abc");		 String s1 = "abc";		String s2 = new String("abc");		 String s3 = "abc";		System.out.println(s == s.intern());		System.out.println(s1 == s2.intern());		System.out.println(s2.intern() == s2.intern());		System.out.println(s3 == s1);	}}
没加final反编译后的代码import java.io.PrintStream;public class ThreadException{  public static void main(String[] paramArrayOfString)  {    String str1 = new String("abc");    String str2 = "abc";    String str3 = new String("abc");    String str4 = "abc";    System.out.println(str1 == str1.intern());    System.out.println(str2 == str3.intern());    System.out.println(str3.intern() == str3.intern());    System.out.println(str4 == str2);  }}

　　是不是都感觉很扯淡,还是C++ Primer里面那句话,对于字面量字符串的处理,有些编译器会保存一个,有些会保存多个副本,所以,

     想判断String字面量是否相等,老老实实的for循环

Java面试题之最扯淡的String


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