java 之方法呼叫 方法傳參 值傳遞還是引用傳遞位元組碼
阿新 • • 發佈:2019-02-08
/*Java中的引數傳遞方式到底是引用傳遞還是值傳遞?java核心技術卷I裡有一個結 論我覺得挺有意思的:java中沒有引用傳遞,只有值傳遞 首先看定義: 值傳遞,是指方法接收的是呼叫者提供的值 引用傳遞,是指方法接收的是呼叫者提供的變數地址 事實上,Java中方法引數傳遞的是原來物件引用的copy(副本)。如果你在方法 中改變這個copy中的內容,因為這個copy也是指向原物件,所以改變會生效。給 你帶來好像Java中存在地址傳遞一樣。而實際上當你對這個物件的引用進行操作 ,例如object = new SomeObject();這樣的操作是無效的,因為你改變的是這個 copy(副本),原來的引用還是指向原來的物件,並沒有發生變化。方法傳參就 相當於這個 */ package com.zhang; public class Test6 { String name = "t"; void change(Test5 t2) { }; void change2(Test5 t2) { t2 = new Test5(); }; public static void main(String[] args) { Test5 t = new Test5(); Test5 t2 = t; /* * //這一步t2就相當於呼叫方法的物件copy副本.就相當於這樣change(Test5 t2)。 t2.name = * "hao";當t2副本物件改變了name屬性的值同時,t的物件name值也改變 了。因為。t,和 副本t2。指向的都是new * Test5();這個物件。當我們企圖change(Test5 t2){t2 = new Test5();} * 沒有改變t2的值是因為函式change(Test5 t2){t2 = new Test5();}中的引用只是原物件引用 * 的一個副本,即t2,並不是t。既然如此,我們在函式中只是改變了引用t2所指向 的物件,而沒有改變引用t所指向的物件。 * 所以當呼叫change(Test5 t2){t2 = new Test5();}方法時候。t2.name結果還是 “好” */ System.out.println(t.name); } } /* * *Compiled from "Test5.java" public * * class Test5 extends java.lang.Object SourceFile: "Test5.java" minor * * version: 0 major version: 50 Constant pool: const #1 = Method #9.#20; * * // java/lang/Object."<init>":()V const #2 = String #21; // t const #3 = * * Field #4.#22; // Test5.name:Ljava/lang/String; const #4 = class #23; // * * Test5 const #5 = Method #4.#20; // Test5."<init>":()V const #6 = String * * #24; // hao const #7 = Field #25.#26; // * * java/lang/System.out:Ljava/io/PrintS tream; const #8 = Method #27.#28; * * // java/io/PrintStream.println:(Ljava/l ang/String;)V const #9 = class * * #29; // java/lang/Object const #10 = Asciz name; const #11 = Asciz * * Ljava/lang/String;; const #12 = Asciz <init>; const #13 = Asciz ()V; * * const #14 = Asciz Code; const #15 = Asciz LineNumberTable; const #16 = * * Asciz main; const #17 = Asciz ([Ljava/lang/String;)V; const #18 = Asciz * * SourceFile; const #19 = Asciz Test5.java; const #20 = NameAndType * * #12:#13;// "<init>":()V const #21 = Asciz t; const #22 = NameAndType * * #10:#11;// name:Ljava/lang/String; const #23 = Asciz Test5; const #24 = * * Asciz hao; const #25 = class #30; // java/lang/System const #26 = * * NameAndType #31:#32;// out:Ljava/io/PrintStream; const #27 = class #33; * * // java/io/PrintStream const #28 = NameAndType #34:#35;// println: * * (Ljava/lang/String;)V const #29 = Asciz java/lang/Object; const #30 = * * Asciz java/lang/System; const #31 = Asciz out; const #32 = Asciz * * Ljava/io/PrintStream;; const #33 = Asciz java/io/PrintStream; const #34 * * = Asciz println; const #35 = Asciz (Ljava/lang/String;)V; { * * java.lang.String name; public Test5(); Code: Stack=2, Locals=1, * * Args_size=1 0: aload_0 1: invokespecial #1; //Method * * java/lang/Object."<init>":()V 4: aload_0 5: ldc #2; //String t 7: * * putfield #3; //Field name:Ljava/lang/String; 10: return * * LineNumberTable: line 1: 0 line 2: 4 public static void main * * (java.lang.String[]); Code: Stack=2, Locals=3, Args_size=1 0: new #4; * * //class Test5 建立一個新物件,並壓入棧頂 3: dup ////複製棧頂數值,並將 * * 複製值壓入棧頂 4: invokespecial #5; //Method "<init>":()V 7: astore_1 * * ////將棧頂引用型別數值存到第二個本地變數(區域性變量表) 8: aload_1 //將 * * 第二個引用型別本地變數推送至棧頂 9: astore_2 //將棧頂引用型別數值存到第 * * 三個本地變數(區域性變量表) 10: aload_2 //將第三個引用型別本地變數推送至 * * 棧頂 11: ldc #6; //String hao 將string hao常量從常量池取出,推入棧頂 * * 13: putfield #3; //Field name:Ljava/lang/String;//為物件的name欄位賦值 * * 16: getstatic #7; //Field java/lang/System.out:Ljava/io/PrintStream; * * 19: aload_1 //將第二個引用型別本地變數推送至棧頂(13步已經把物件引用物件 * * name地段賦值了) 20: getfield #3; //Field name:Ljava/lang/String; //獲取 * * 物件的相應欄位name。並壓入棧頂 23: invokevirtual #8; //Method * * java/io/PrintStream.println:(Ljava/lang/Str ing;)V 26: return * * LineNumberTable: line 4: 0 line 5: 8 line 6: 10 line 7: 16 line 8: 26 } */
package com.zhang; public class Test2{ public static void changeStr(String str){ str="welcome"; } public static void main(String[] args) { String str="1234"; str = "welcome"; // changeStr(str); System.out.println(str); } } /** *Compiled from "Test2.java" public class Test2 extends java.lang.Object SourceFile: "Test2.java" minor version: 0 major version: 50 Constant pool: const #1 = Method #7.#18; // java/lang/Object."<init>":()V const #2 = String #19; // welcome const #3 = String #20; // 1234 const #4 = Field #21.#22; // java/lang/System.out:Ljava/io/PrintS tream; const #5 = Method #23.#24; // java/io/PrintStream.println:(Ljava/l ang/String;)V const #6 = class #25; // Test2 const #7 = class #26; // java/lang/Object const #8 = Asciz <init>; const #9 = Asciz ()V; const #10 = Asciz Code; const #11 = Asciz LineNumberTable; const #12 = Asciz changeStr; const #13 = Asciz (Ljava/lang/String;)V; const #14 = Asciz main; const #15 = Asciz ([Ljava/lang/String;)V; const #16 = Asciz SourceFile; const #17 = Asciz Test2.java; const #18 = NameAndType #8:#9;// "<init>":()V const #19 = Asciz welcome; const #20 = Asciz 1234; const #21 = class #27; // java/lang/System const #22 = NameAndType #28:#29;// out:Ljava/io/PrintStream; const #23 = class #30; // java/io/PrintStream const #24 = NameAndType #31:#13;// println:(Ljava/lang/String;)V const #25 = Asciz Test2; const #26 = Asciz java/lang/Object; const #27 = Asciz java/lang/System; const #28 = Asciz out; const #29 = Asciz Ljava/io/PrintStream;; const #30 = Asciz java/io/PrintStream; const #31 = Asciz println; { public Test2(); 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 public static void changeStr(java.lang.String); Code: Stack=1, Locals=1, Args_size=1 0: ldc #2; //String welcome 將string welcome常量從常量池取出,推入棧頂 2: astore_0 //將棧頂引用型別數值存到第一個本地變數(區域性變量表) 3: return LineNumberTable: line 3: 0 line 4: 3 public static void main(java.lang.String[]); Code: Stack=2, Locals=2, Args_size=1 0: ldc #3; //String 1234 將string 1234常量從常量池取出,推入棧頂 2: astore_1 //將棧頂引用型別數值存到第二個本地變數(區域性變量表) 3: ldc #2; //String welcome 將string welcome常量從常量池取出,推入棧頂 5: astore_1 //將棧頂引用型別數值存到第二個本地變數(區域性變量表) 6: getstatic #4; //Field java/lang/System.out:Ljava/io/PrintStream; 9: aload_1 //將第二個引用型別本地變數推送至棧頂 10: invokevirtual #5; //Method java/io/PrintStream.println:(Ljava/lang/Str ing;)V 13: return LineNumberTable: line 6: 0 line 7: 3 line 9: 6 line 10: 13 } * **/
package com.zhang; public class Test4{ public static void changeStr(T str){ str.name = "sf"; } public static void main(String[] args) { T t = new T(); changeStr(t); System.out.println(t.name); } } class T{ String name = "name--min"; @Override public String toString() { // TODO Auto-generated method stub return name; } } /* *Compiled from "Test4.java" public class Test4 extends java.lang.Object SourceFile: "Test4.java" minor version: 0 major version: 50 Constant pool: const #1 = Method #10.#21; // java/lang/Object."<init>":()V const #2 = String #22; // sf const #3 = Field #4.#23; // T.name:Ljava/lang/String; const #4 = class #24; // T const #5 = Method #4.#21; // T."<init>":()V const #6 = Method #9.#25; // Test4.changeStr:(LT;)V const #7 = Field #26.#27; // java/lang/System.out:Ljava/io/Print tream; const #8 = Method #28.#29; // java/io/PrintStream.println:(Ljava/ ang/String;)V const #9 = class #30; // Test4 const #10 = class #31; // java/lang/Object const #11 = Asciz <init>; const #12 = Asciz ()V; const #13 = Asciz Code; const #14 = Asciz LineNumberTable; const #15 = Asciz changeStr; const #16 = Asciz (LT;)V; const #17 = Asciz main; const #18 = Asciz ([Ljava/lang/String;)V; const #19 = Asciz SourceFile; const #20 = Asciz Test4.java; const #21 = NameAndType #11:#12;// "<init>":()V const #22 = Asciz sf; const #23 = NameAndType #32:#33;// name:Ljava/lang/String; const #24 = Asciz T; const #25 = NameAndType #15:#16;// changeStr:(LT;)V const #26 = class #34; // java/lang/System const #27 = NameAndType #35:#36;// out:Ljava/io/PrintStream; const #28 = class #37; // java/io/PrintStream const #29 = NameAndType #38:#39;// println:(Ljava/lang/String;)V const #30 = Asciz Test4; const #31 = Asciz java/lang/Object; const #32 = Asciz name; const #33 = Asciz Ljava/lang/String;; const #34 = Asciz java/lang/System; const #35 = Asciz out; const #36 = Asciz Ljava/io/PrintStream;; const #37 = Asciz java/io/PrintStream; const #38 = Asciz println; const #39 = Asciz (Ljava/lang/String;)V; { public Test4(); 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 public static void changeStr(T); Code: Stack=2, Locals=1, Args_size=1 0: aload_0 ////將棧頂引用型別數值存到第一個本地變數(區域性變量表) 1: ldc #2; //String sf 將string sf常量從常量池取出,推入棧頂 3: putfield #3; //Field T.name:Ljava/lang/String;//為t物件的name欄位賦值 6: return LineNumberTable: line 3: 0 line 4: 6 public static void main(java.lang.String[]); Code: Stack=2, Locals=2, Args_size=1 0: new #4; //class T 建立一個新物件,並壓入棧頂 3: dup //複製棧頂數值,並將複製值壓入棧頂 4: invokespecial #5; //Method T."<init>":()V 調入無參夠造方法 7: astore_1 //將棧頂引用型別數值存到第二個本地變數(區域性變量表) 8: aload_1 //將第二個引用型別本地變數推送至棧頂 9: invokestatic #6; //Method changeStr:(LT;)V 12: getstatic #7; //Field java/lang/System.out:Ljava/io/PrintStream; 15: aload_1 //將第二個引用型別本地變數推送至棧頂 16: getfield #3; //Field T.name:Ljava/lang/String; //獲取物件的相應欄位name。並壓入棧頂 19: invokevirtual #8; //Method java/io/PrintStream.println:(Ljava/lang/St ing;)V 22: return LineNumberTable: line 7: 0 line 8: 8 line 9: 12 line 10: 22 } * */
- 下載次數: 1