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Compile-time and Runtime Constants

Please learn from the following code first

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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:

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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:

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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.

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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");
}
}
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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();
}
}