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資料型別
裝箱：把基本型別用它們對應的引用型別包裝起來，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，將引用類型的物件簡化成值類型的資料
Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i)) int b = new Integer(100); 这是自动拆箱
列印結果是什麼？
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));
}
}
「==」比較的是位址，而a和b兩個物件的位址不同，即是兩個對象，所以都是false
透過javap解析字節碼，內容如下
m11 result false m12 result false
m2
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/In teger; 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/la ng/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/la ng/StringBuilder; 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)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/In teger; 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/la ng/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/la ng/StringBuilder; 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 49: return </init></init></init></init>
〜＜
javap解析內容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));
}
}m21 result false m22 result false
列印結果
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/la ng/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/la ng/StringBuilder; 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 51: return public 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/la ng/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/la ng/StringBuilder; 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 53: return
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));
}
}m31 result true m32 result false
public void m31(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 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/la ng/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/la ng/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 43: return public void m32(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 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/la ng/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/la ng/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 45: return
javap解析內容
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));
}
}
分析
javap是Java自帶的一個工具，可以反編譯，也可以查看Java編譯器產生的字節碼(上面程式碼只使用了javap -c DataType)，是分析程式碼的一個好工具，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是同一個物件。
但a對像是呼叫Integer.valueOf()產生的，b是透過自動裝箱產生的對象，為什麼會是同一個對象呢？再看一下字節碼吧，畢竟Java程式是靠虛擬機器來運行字節碼來實現的。
m41這個方法只適用了一次valueOf()，但字節碼中出現了兩次，表示自動裝箱時也呼叫了valueOf()。
下面是valueOf()具體實作
m41 result true m42 result false
在【-128，127】之間的數字，valueOf回傳的是快取中的物件，所以兩次呼叫返回的是同一個物件。
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