Data Types

Primitive

Primitive data types are byte, short, int long, float, double, char and boolean. Both of them have a reference type class defined.

Type	Native Type	Bytes	Bits	Default	Range	Related Reference Class	Reference Class Cache Range	Configurable
byte	jbyte	1	8	0	[-128, 127]	Byte	[-128, 127]	N
int	jint	4	32	0	[\(-2^{-32}\), \(2^{32}-1\)]	Integer	[-128, 127]	Y -XX:AutoBoxCacheMax=N
float	jfloat	4	32	0f		Float
double	jdouble	8	64	0d		Double
char	jchar	2	u16	'u0000'		Character	[0, 127]	N
boolean	jboolean	1	u8	false		Boolean	[false, true]	N
short	jshort	2	16	0	[\(-2^{16}\), \(2^{16}-1\)]	Short
long	jlong	8	64	0l	[\(-2^{64}\), \(2^{64} - 1\)]	Long
void	void		N/A	N/A		Void

Reference

• Class
• Interface
• Array Class
• Generic（Generic Parameter will be eased during the compile）

Pass by value OR Pass by reference

Primitive Type

Take the following code as an example,

Java

public class Main {
    private static void test(int a) {
        System.out.println(a);
        // try to modify a 
        a = 1000;
    }

    public static void main(String[] args) {
        int a = 10;
        test(a);
        System.out.println(a);
    }
}

The output:
10
10

From the result we can see it pass the variable by "value".

• in method main the variable a is allocated in stack
• when pass to the method test, the a is copied, so when change the value of a in method test, it will not change the value in main

Reference Type

Take the following code as an example,

Java

public class Main {
    private static void test1(AtomicInteger a1) {
        System.out.println(a1.get());
        // try to modify a1 
        a1 = new AtomicInteger(10000);
    }

    private static void test2(AtomicInteger a2) {
        // try to modify a2
        a2.set(1000);
    }

    public static void main(String[] args) {
        AtomicInteger a = new AtomicInteger(10);
        test1(a);
        System.out.println(a.get());

        test2(a);
        System.out.println(a.get());
    }
}

The output:
10
10
1000

From the result we can see the it doesn't pass the variable by reference.

• a is a variable allocated in stack reference to a heap memory which stores the value 10
• when pass to method test1, it tries to assign a new variable to the passed variable a1, but the origin a is not changed This is because a copied a which is a1 passed to method test1, we changed the reference of a1 to a new memory address, but no effect to the origin referenced memory
• when pass to method test2, it tries to set a new value for the origin object, so in main method prints the new value. Although the a2 is a copied variable, it has the same reference memory, so the changes reflect in main.