Java 裝箱與拆箱詳解及實例代碼

Java 裝箱與拆箱詳解

前言：

要理解裝箱和拆箱的概念，就要理解Java數據類型

裝箱：把基本類型用它們相應的引用類型包裝起來，使其具有對象的性質。 int包裝成Integer、float包裝成Float

拆箱：和裝箱相反，將引用類型的對像簡化成值類型的數據

Integer a = 100; 這是自動裝箱(編譯器調用的是static Integer valueOf(int i))int b = new Integer(100); 這是自動拆箱

看下面一段代碼

m1

 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m11(); dt.m12(); } public void m11() { Integer a = new Integer(100); Integer b = 100; System.out.println("m11 result " + (a == b)); } public void m12() { Integer a = new Integer(128); Integer b = 128; System.out.println("m12 result " + (a == b)); } }

打印結果是什麼？

 m11 result falsem12 result false

“==”比較的是地址，而a和b兩個對象的地址不同，即是兩個對象，所以都是false

通過javap解析字節碼，內容如下

public void m11(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 9: astore_1 10: bipush 100 12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 15: astore_2 16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 19: new #59; //class java/lang/StringBuilder 22: dup 23: ldc #61; //String m11 result 25: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 28: aload_1 29: aload_2 30: if_acmpne 37 33: iconst_1 34: goto 38 37: iconst_0 38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 47: return public void m12(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 10: astore_1 11: sipush 128 14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 17: astore_2 18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 21: new #59; //class java/lang/StringBuilder 24: dup 25: ldc #82; //String m12 result 27: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 30: aload_1 31: aload_2 32: if_acmpne 39 35: iconst_1 36: goto 40 39: iconst_0 40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 49: return</init></init></init></init>

m2

 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m21(); dt.m22(); } public void m21() { Integer a = new Integer(100); Integer b = new Integer(100); System.out.println("m21 result " + (a == b)); } public void m22() { Integer a = new Integer(128); Integer b = new Integer(128); System.out.println("m22 result " + (a == b)); } }

打印結果是

m21 result falsem22 result false

a和b仍是兩個對象

javap解析內容

public void m21(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 9: astore_1 10: new #44; //class java/lang/Integer 13: dup 14: bipush 100 16: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 19: astore_2 20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 23: new #59; //class java/lang/StringBuilder 26: dup 27: ldc #84; //String m21 result 29: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 32: aload_1 33: aload_2 34: if_acmpne 41 37: iconst_1 38: goto 42 41: iconst_0 42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 51: returnpublic void m22(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 10: astore_1 11: new #44; //class java/lang/Integer 14: dup 15: sipush 128 18: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 21: astore_2 22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 25: new #59; //class java/lang/StringBuilder 28: dup 29: ldc #86; //String m22 result 31: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 34: aload_1 35: aload_2 36: if_acmpne 43 39: iconst_1 40: goto 44 43: iconst_0 44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 53: return

m3

 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m31(); dt.m32(); } public void m31() { Integer a = 100; Integer b = 100; System.out.println("m31 result " + (a == b)); } public void m32() { Integer a = 128; Integer b = 128; System.out.println("m32 result " + (a == b)); } }

打印結果

m31 result truem32 result false

為什麼有第一個是true，第二個是false呢？觀察javap解析的數據

javap解析內容

public void m31(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #88; //String m31 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 43: returnpublic void m32(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #90; //String m32 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 45: return

m4

 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m41(); dt.m42(); } public void m41() { Integer a = Integer.valueOf(100); Integer b = 100; System.out.println("m41 result " + (a == b)); } public void m42() { Integer a = Integer.valueOf(128); Integer b = 128; System.out.println("m42 result " + (a == b)); }}

打印結果

m41 result truem42 result false

javap解析內容

public void m41(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #92; //String m41 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 43: returnpublic void m42(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #94; //String m42 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 45: return}

分析

javap是Java自帶的一個工具，可以反編譯，也可以查看Java編譯器生成的字節碼(上面代碼只使用了javap -c DataType)，是分析代碼的一個好工具，具體怎麼使用請Google一下

先看一下m4，為什麼運行結果中出現了“true”呢，true說明a、b是同一個對象。

但a對像是調用Integer.valueOf()生成的，b是通過自動裝箱生成的對象，為什麼會是同一個對象呢？再看一下字節碼吧，畢竟Java程序是依靠虛擬機運行字節碼實現的。

m41這個方法只適用了一次valueOf()，但字節碼中出現了兩次，說明自動裝箱時也調用了valueOf()。

下面是valueOf()具體實現

/** * Returns a <tt>Integer</tt> instance representing the specified * <tt>int</tt> value. * If a new <tt>Integer</tt> instance is not required, this method * should generally be used in preference to the constructor * {@link #Integer(int)}, as this method is likely to yield * significantly better space and time performance by caching * frequently requested values. * * @param i an <code>int</code> value. * @return a <tt>Integer</tt> instance representing <tt>i</tt>. * @since 1.5 */public static Integer valueOf(int i) {final int offset = 128;if (i >= -128 && i <= 127) { // must cache return IntegerCache.cache[i + offset];} return new Integer(i);}

在【-128，127】之間的數字，valueOf返回的是緩存中的對象，所以兩次調用返回的是同一個對象。

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