Explanation of several points needing attention in final (tested by the author)

First we test final Impact on non static variables and methods

Yes final Under the circumstances :

public class FinalTest {
    final int a = 1;
    final String s = "sss";
    final void haha() {
        System.out.println("haha");
        System.out.println(a);
    }
    public static void main(String[] args) {
        FinalTest finalTest = new FinalTest();
    }
}

Decompile bytecode file :

public class com.heaboy.mvc.FinalTest {
  final int a;

  final java.lang.String s;

  public com.heaboy.mvc.FinalTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_1
       6: putfield      #2                  // Field a:I
       9: aload_0
      10: ldc           #3                  // String sss
      12: putfield      #4                  // Field s:Ljava/lang/String;
      15: return

  final void haha();
    Code:
       0: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #6                  // String haha
       5: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      11: iconst_1
      12: invokevirtual #9                  // Method java/io/PrintStream.println:(I)V
      15: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #8                  // class com/heaboy/mvc/FinalTest
       3: dup
       4: invokespecial #10                 // Method "<init>":()V
       7: astore_1
       8: return
}

nothing final Under the circumstances :

public class FinalTest {
    int a = 1;
    String s = "sss";
    void haha() {
        System.out.println("haha");
        System.out.println(a);
    }
    public static void main(String[] args) {
        FinalTest finalTest = new FinalTest();
    }
}

Decompile bytecode file :

public class com.heaboy.mvc.FinalTest {
  int a;

  java.lang.String s;

  public com.heaboy.mvc.FinalTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_1
       6: putfield      #2                  // Field a:I
       9: aload_0
      10: ldc           #3                  // String sss
      12: putfield      #4                  // Field s:Ljava/lang/String;
      15: return

  void haha();
    Code:
       0: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #6                  // String haha
       5: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
      11: aload_0
      12: getfield      #2                  // Field a:I
      15: invokevirtual #8                  // Method java/io/PrintStream.println:(I)V
      18: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #9                  // class com/heaboy/mvc/FinalTest
       3: dup
       4: invokespecial #10                 // Method "<init>":()V
       7: astore_1
       8: return
}

All in all ,final In both haha() There is a little difference in the method :

• What we output here is a:

  • No, final Decorated variable a, We need to aload_0,getfield, That is, load variables , Get the value of the variable , Then the output

  • Yes final Decorated variable a, We just need to load iconst_1 perhaps sipush( This is a big number , For example, we give a The initial assignment is 10, This is big number ,jvm You don't use it iconst_1, Instead, choose to use sipush To perform the operation of assignment ), That is, directly load constants , Direct output
  • Theoretically final The modified constant call is faster

---

Then we test final Yes Static variables and methods Influence

Yes final Under the circumstances :

public class FinalTest {
    final static int a = 1;
    final static String s = "sss";
    final static void haha() {
        System.out.println(a);
    }
    public static void main(String[] args) {
        FinalTest finalTest = new FinalTest();
    }
}

Decompile bytecode file :

public class com.heaboy.mvc.FinalTest {
  static final int a;

  static final java.lang.String s;

  public com.heaboy.mvc.FinalTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  static final void haha();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: iconst_1
       4: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
       7: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #3                  // class com/heaboy/mvc/FinalTest
       3: dup
       4: invokespecial #5                  // Method "<init>":()V
       7: astore_1
       8: return
}

No, final Under the circumstances :

public class FinalTest {
    static int a = 1;
    static String s = "sss";
    static void haha() {
        System.out.println(a);
    }
    public static void main(String[] args) {
        FinalTest finalTest = new FinalTest();
    }
}

Decompile bytecode file :

public class com.heaboy.mvc.FinalTest {
  static int a;

  static java.lang.String s;

  public com.heaboy.mvc.FinalTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  static void haha();
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: getstatic     #3                  // Field a:I
       6: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
       9: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #5                  // class com/heaboy/mvc/FinalTest
       3: dup
       4: invokespecial #6                  // Method "<init>":()V
       7: astore_1
       8: return

  static {};
    Code:
       0: iconst_1
       1: putstatic     #3                  // Field a:I
       4: ldc           #7                  // String sss
       6: putstatic     #8                  // Field s:Ljava/lang/String;
       9: return
}

All in all ,final The variables decorated are different , The two haha() There is also a little difference in the method :

• final The decorated variable is not in the static area , But in the constant pool

• What we output here is a:

  • No, final Decorated variable a, We need to getstatic, That is, load static variables , Then the output

  • Yes final Decorated variable a, We just need to load iconst_1 perhaps sipush That is, directly load constants , Direct output

---

Father son inheritance

1. final Modified classes cannot be inherited
2. final Decorated objects and arrays , We can modify the contents , But you can't change the address
3. We cannot re assign values to variables of the parent class , And cannot override the method of the parent class

We test the first case : Will report a mistake , Cannot inherit final Modified class

public class FinalTest extends FinalDadTest {
    
}

final class FinalDadTest {

}

 

When we test the second case, we will output 1111: We can figure out ,final Decorated objects and arrays , We can modify the contents , But you can't change the address

public class FinalTest extends FinalDadTest {
    public static void main(String[] args) {
        final FinalTest finalTest = new FinalTest();
        final int[] arr = finalTest.arr;
        arr[0] = 1111;
        System.out.println(arr[0]);//  Output 1111
    }
}

class FinalDadTest {
    final int a = 1;
    final int[] arr = new int[10];
    final void haha() {
        System.out.println("sss");
    }
    final void cece() {
        System.out.println("cece");
    }
}

 

We test the third case : We will find that we cannot give the parent class a Reassign , And cannot override the method of the parent class haha().

public class FinalTest extends FinalDadTest {
    public FinalTest() {
        super.a = 2;
    }
    void haha() {
        System.out.println(a);
    }
    public static void main(String[] args) {
    }
}

class FinalDadTest {
    final int a = 1;
    final void haha() {
        System.out.println("sss");
    }
}

end