Compile-time and Runtime Constants

Please learn from the following code first

package individual.cy.learn.mess;

/**
 * @author mystic
 */
public class CompileAndRuntimeConstant {
    public static void main(String[] args) {
        // No load static block
        System.out.println(Test.NAME);
        // No load static block
        System.out.println(Test.SCORE);
        // No load static block
        System.out.println(Test.PI);
        // load static block
        System.out.println(Test.CC);
        // load static block(cc and age only load once totally)
        System.out.println(Test.age);
        System.out.println();
    }
}

class Test {
    /**
     * The following three: compile-time constants
     */
    public static final String NAME = "shanxi";
    public static final int SCORE = 85;
    public static final double PI = 3.1415;

    /**
     * The following two: runtime-constants
     */
    public static final Integer CC = 2;
    public static int age = 23;

    static {
        System.out.println("Test static block: " + PI);
    }

    /**
     * The following one: compile-time constant
     */
    private final int gg = 10;
    private int hh = gg;

    public int getHh() {
        return hh;
    }

    public void setHh(int hh) {
        this.hh = hh;
    }
}

Its outputs is as follows:

shanxi
85
3.1415
Test static block: 3.1415
2
23

compile-time constant

Compile-time Constant: A variable use final modifier, its type is primitive type or String, and its value is a constant expression.

What is constant expression?

Constant Expressions in Oracle

e.g. Constant expressions:

true
(short)(1*2*3*4*5*6)
Integer.MAX_VALUE / 2
2.0 * Math.PI
"The integer " + Long.MAX_VALUE + " is mighty big."

A constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the
following:

• Literals of primitive type and literals of type String

• Casts to primitive types and casts to type String

• The unary operators +, -, ~, and ! (but not ++ or --)

• The multiplicative operators *, /, and %

• The additive operators + and -

• The shift operators <<, >>, and >>>

• The relational operators <, <=, >, and >= (but not instanceof)

• The equality operators == and !=

• The bitwise and logical operators &, ^, and |

• The conditional-and operator && and the conditional-or operator ||

• The ternary conditional operator ? :

• Parenthesized expressions whose contained expression is a constant expression.

• Simple names that refer to constant variables.

• Qualified names of the form TypeName . Identifier that refer to constant variables.

Constant expressions of type String are always "interned" so as to share unique instances, using the method String.intern.

A constant expression is always treated as FP-strict, even if it occurs in a context where a non-constant expression would not be considered to be
FP-strict.

All in all, except for instanceof, ++, --

runtime constant

Runtime constant: A variable use static modifier, except for compile-time constant.

Class file

Compile-time constants' reference will be replaced with the actual value, like as follows.

package individual.cy.learn.mess;

class Test {
    public static final String NAME = "shanxi";
    public static final int SCORE = 85;
    public static final double PI = 3.1415D;
    public static final Integer CC = 2;
    public static int age = 23;
    private final int gg = 10;
    private int hh = 10;

    Test() {
    }

    public int getHh() {
        return this.hh;
    }

    public void setHh(int hh) {
        this.hh = hh;
    }

    static {
        System.out.println("Test static block: 3.1415");
    }
}

package individual.cy.learn.mess;

public class CompileAndRuntimeConstant {
    public CompileAndRuntimeConstant() {
    }

    public static void main(String[] args) {
        System.out.println("shanxi");
        System.out.println(85);
        System.out.println(3.1415D);
        System.out.println(Test.CC);
        System.out.println(Test.age);
        System.out.println();
    }
}