Java 박싱 및 언박싱에 대한 자세한 설명과 예제 코드

Java boxing 및 unboxing에 대한 자세한 설명

머리말:

boxing 및 unboxing의 개념을 이해하려면 Java 데이터 유형을 이해해야 합니다.

Boxing: 기본 유형은 해당 참조 유형으로 래핑되어 객체의 속성을 제공합니다. Int는 Integer로 패키징되고, float는 Float로 패키징됩니다

Unboxing: boxing과 달리 참조 유형 객체는 값 유형 데이터로 단순화됩니다

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

다음 코드를 보세요

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));
  }
  
    
}

인쇄 결과는 무엇인가요?

m11 result false
m12 result false

"==" 주소를 비교한 결과 두 객체 a와 b의 주소가 다르다는 것, 즉 두 객체이므로 둘 다 거짓

javap을 통해 바이트코드를 구문 분석하면 내용은 다음과 같습니다

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>

　

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));
  }
  
    
}

인쇄된 결과는

m21 result false
m22 result false

a와 b는 여전히 두 객체입니다

javap 구문 분석된 콘텐츠

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

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));
  }
  
  
}

인쇄 결과

m31 result true
m32 result false

왜 첫 번째는 참이고 두 번째는 거짓인가요? javap

javap parsed content

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

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));
  }
}

결과 인쇄

로 파싱된 데이터 관찰

m41 result true
m42 result false

javap 분석 콘텐츠

public void m41();
 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   #92; //String m41 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 m42();
 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   #94; //String m42 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
 
}

분석

javap은 Java와 함께 제공됩니다. Java 컴파일러에서 생성된 바이트코드를 볼 수 있습니다(위 코드는 javap -c DataType만 사용함). 구체적인 사용 방법은 Google에서 확인하세요. 먼저 m4를 보면, 실행 결과에 "true"가 나타나는 이유는 무엇입니까? True는 a와 b가 동일한 개체라는 것을 의미합니다.

그런데 객체 a는 Integer.valueOf()를 호출하여 생성되고, 객체 b는 자동 박싱에 의해 생성되는 이유는 무엇입니까? 바이트코드를 다시 살펴보겠습니다. 결국 Java 프로그램은 바이트코드를 실행하기 위해 가상 머신에 의존하여 구현됩니다.

m41 이 메서드는 valueOf()를 한 번만 적용하지만 바이트코드에 두 번 나타나며 이는 autoboxing 중에 valueOf()도 호출된다는 것을 나타냅니다.

다음은 valueOf()의 구체적인 구현입니다.

/**
 * 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);
}

[-128, 127] 사이의 숫자, valueOf는 캐시 객체를 반환합니다. 이므로 두 호출은 동일한 객체를 반환합니다.

읽어주셔서 감사합니다. 도움이 되기를 바랍니다. 이 사이트를 지원해 주셔서 감사합니다!

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