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));
  }
  
    
}

   

m3

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

   

為什麼有第一個是true，第二個是false呢？觀察javap解析的資料

javap解析內容

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));
  }
  
  
}

   

m4

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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