java八股文面试[java基础]—— 重载和重写

冷不防 2024-03-24 18:10 76阅读 0赞

![87fd630d882d4c429bcf6c3c06589099.png][]

![ef2017b5edee4f479213c81105f09e63.png][]

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Overload Override Overwrite三者区别：

![1d0ae764ee5c46698f546eb2ba08c9bb.png][]

Java中，有一个名叫方法签名的东西，它的定义是这样的

Definition: Two of the components of a method declaration comprise the method signature—the method’s name and the parameter types.

大概意思是，方法签名有两部分组成——方法名和参数列表

我上面的函数的方法签名就是 hand(int a,int b) 而简单来说，JVM就是通过方法签名来决定去调用哪一个方法的，而从方法签名的组成部分来说，返回类型并不在方法签名中，所以当一个类中出现方法名参数列表相同，只有返回类型不同时，JVM无法根据方法签名来确定该调用哪一个方法，所以会报错。

参考：https://blog.csdn.net/qq\_53574615/article/details/130379554

扩展：

可以通过修改字节码，把函数修改成同名：

构造两个不同名字 相同参数的成员函数：

public class OverWrite {
        public String getString(String str) {
            return str;
        }
    
        public void getString1(String str) {
            return;
        }
    }

通过javap -v 命令查看字节码：

D:\Program Files\Java\jdk1.8.0_331\bin>javap.exe -v D:\project\test\out\production\test\OverWrite.class
    Classfile /D:/project/test/out/production/test/OverWrite.class
      Last modified 2023-8-17; size 504 bytes
      MD5 checksum 40c07ae9b391609cd8fb00b3d2924ce2
      Compiled from "OverWrite.java"
    public class OverWrite
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #3.#19         // java/lang/Object."<init>":()V
       #2 = Class              #20            // OverWrite
       #3 = Class              #21            // java/lang/Object
       #4 = Utf8               <init>
       #5 = Utf8               ()V
       #6 = Utf8               Code
       #7 = Utf8               LineNumberTable
       #8 = Utf8               LocalVariableTable
       #9 = Utf8               this
      #10 = Utf8               LOverWrite;
      #11 = Utf8               getString
      #12 = Utf8               (Ljava/lang/String;)Ljava/lang/String;
      #13 = Utf8               str
      #14 = Utf8               Ljava/lang/String;
      #15 = Utf8               getString1
      #16 = Utf8               (Ljava/lang/String;)V
      #17 = Utf8               SourceFile
      #18 = Utf8               OverWrite.java
      #19 = NameAndType        #4:#5          // "<init>":()V
      #20 = Utf8               OverWrite
      #21 = Utf8               java/lang/Object
    {
      public OverWrite();
        descriptor: ()V
        flags: ACC_PUBLIC
        Code:
          stack=1, locals=1, args_size=1
             0: aload_0
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             4: return
          LineNumberTable:
            line 1: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   LOverWrite;
    
      public java.lang.String getString(java.lang.String);
        descriptor: (Ljava/lang/String;)Ljava/lang/String;
        flags: ACC_PUBLIC
        Code:
          stack=1, locals=2, args_size=2
             0: aload_1
             1: areturn
          LineNumberTable:
            line 3: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       2     0  this   LOverWrite;
                0       2     1   str   Ljava/lang/String;
    
      public void getString1(java.lang.String);
        descriptor: (Ljava/lang/String;)V
        flags: ACC_PUBLIC
        Code:
          stack=0, locals=2, args_size=2
             0: return
          LineNumberTable:
            line 7: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       1     0  this   LOverWrite;
                0       1     1   str   Ljava/lang/String;
    }
    SourceFile: "OverWrite.java"

可以看到getString 和 getString1 的两个函数定义

![b19b7536d9944d0faf204091ffdc1f72.png][]

可以通过字节码修改工具把getString1改成getString，实际class还是可以跑的。

Main函数中记录了调用的返回值信息：

![30f1672b1a2c4c34836bb44068079a4d.png][]

所以即使OverWrite.class中有两个同名不同返回值的函数，JVM可以进行正常跑，只不过java编译器做了如此限定。

知识来源：

[【基础】重载和重写的区别\_哔哩哔哩\_bilibili][bilibili]

[【2023年面试】Java中重写和重载的区别\_哔哩哔哩\_bilibili][2023_Java_bilibili]

[【23版面试突击】Overload、Override、Overwrite的区别\_哔哩哔哩\_bilibili][23_Overload_Override_Overwrite_bilibili]

[87fd630d882d4c429bcf6c3c06589099.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/d1d5025ff0c947f4b800a92209bc3eec.png
[ef2017b5edee4f479213c81105f09e63.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/fd5a8dc64b544e7fbd9d8c11c24c34dd.png
[cca443b1f6df4306bbd76e76c6740689.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/dcac566471e64c24bd947c28792ac74e.png
[1d0ae764ee5c46698f546eb2ba08c9bb.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/c400e25394b44b22a16c9448b9144495.png
[b19b7536d9944d0faf204091ffdc1f72.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/ba6dc48aa98d4c6b8aeef0602bff30dd.png
[30f1672b1a2c4c34836bb44068079a4d.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/13/5ffcdfbbd2a54e21839402d636a1e2de.png
[bilibili]: https://www.bilibili.com/video/BV1M24y1z7BR?p=6&vd_source=7ec3a2ec18181a9b64c9a3246305e511
[2023_Java_bilibili]: https://www.bilibili.com/video/BV1Vb4y177wm?p=12&vd_source=7ec3a2ec18181a9b64c9a3246305e511
[23_Overload_Override_Overwrite_bilibili]: https://www.bilibili.com/video/BV1M24y1z7BR?p=134&vd_source=7ec3a2ec18181a9b64c9a3246305e511