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
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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();
}
}
|