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.