JVM instruction set collation

With the above instruction set table, it is much easier to look at the bytecode.

Corresponding English = > en.wikipedia.org/wiki/Java_b…

To a few sample:

sample1

• java

public void sample1(){
            int num = 5;
            }
        Copy the code

• Javap -c View bytecode

public void sample1();
            Code:
               0: iconst_5
               1: istore_1
               2: return
               Copy the code

• explain

Iconst_5 // pushes int 5 to the top of the stack istore_1 // stores the top int to the first local variable return // returns void from the current methodCopy the code

sample2

• java

public int sample2(int a, int b) {
            return a + b;
            }
        Copy the code

• Bytecode and interpretation

public int sample2(int, int); Code: 0: iload_1 // push the first int to the top of the stack 1: iload_2 // push the second int to the top of the stack 2: iadd // Add the two int values at the top of the stack and push the result to the top of the stack 3: Ireturn // Returns an int from the current methodCopy the code

sample3

• java

A little bit more complicated

public float sample3() {
            float num = 0;
            for (int i = 0; i < 5; i++) {
                num *= i;
            }
            return num;
            }
        Copy the code

• Bytecode and interpretation

public float sample3(); Code: 0: fconst_0 // Push float 0 to the top of the stack 1: fstore_1 // Store float value to the first local variable 2: iconst_0 // Push int 0 to the top of the stack, that is, I = 0 in the for loop 3: Istore_2 // store the top int in the second local variable 4: iload_2 // push the second local int to the top of the stack 5: iconst_5 // push the int 5 to the top of the stack, which is the maximum value in the for loop 5 6: If_icmpge 20 // compare the size of a stack int (0) and the size of a stack int (0); // compare the size of a stack int (0) and the size of a stack int (0). Fload_1 // Pushes the first local variable of type float to the top of the stack, num 10: iload_2 // Pushes the second local variable of type int to the top of the stack, which is I 11 in the for loop: I2f = i2f = i2f = i2f = i2f = i2f = i2f = i2f Fstore_1 // Store the top stack float value in the first local variable, which is the product of I and num 14: Iinc 2, 1; iinc 2, 1; iinc 2, 1; iinc 2, 1; iINC 2, 1; iINC 2, 1; iINC 2, 1; Fload_1 // Pushes the first local variable of type float to the top of the stack 21: freturn // Returns float from the current methodCopy the code

These are three simple examples, and the reader can begin to see why switch is more efficient than if else, starting with bytecode.