Java Quick Reference - Language Fundamentals

Language Fundamentals

Operators and Assignments

Flow Control and Exceptions

Declarations and Access Control

Garbage Collection

Overloading and Overriding

Threads

The java.lang Package

The java.util Package

The java.awt Package

The java.io Package

References

Miscellaneous Notes

Tips & Traps

Mock Exams

Language Fundamentals - Default values

Type	Default value
boolean	false
byte	0
char	'\u0000'
short	0
int	0
long	0l
float	0.0f
double	0.0d
Object	null
Array	based on Array type

Automatic Initialization

Field variables (class members) are automatically initialized to default values
Local variables (method or constructor variables) are not automatically initialized

Arrays, whether field or local variables, are automatically initialized to the default values of their declared type


class CheckInit {

    // field variable
    static int i;                         
    // field array reference variable
    static String[] s = new String[10];   

    static void myMethod(){

        int j;                // local variable
        int[] a = new int[5]; // local variable array

        // causes compile error if not explicitly initialized
        j = 10;     

        System.out.println("  Local variable: " + j);
        System.out.println(" Local array ref: " + a[3]);
    }

    public static void main(String[] args) {
        System.out.println("Field variable i: " + i);
        System.out.println(" Field array ref: " + s[2]);
        myMethod();
    }
}

Output of CheckInit:

    Field variable i: 0         // default value of int
     Field array ref: null      // default value for String[]
      Local variable: 10        // explicit value
     Local array ref: 0         // default value of int[]

Timing and duration of variable initializations (JLS §4.5.3)

Variable Type	Definition	Initialization
Class (Field)	Declared with the static keyword within a class or interface	Created when the class or interface is prepared. Automatically initialized to the default value of its type Duration: as long as the class is loaded
Instance (Field)	Declared within a class without the keyword static	Created when a new instance is created Automatically initialized to the default value of its type Duration: for the life of the instance object
Array components	unnamed variables created when an array object is created not when declared	intialized to the default value of the array type Duration: until the array is no longer referenced
Method parameters	named argument values passed to a method	a new parameter variable is created each time the method is invoked initialized with the corresponding argument value from the method call Duration: method execution
Constructor parameters	named argument values passed to the constructor	a new parameter variable is created each time a new instance is created or the constructor is called initialized to the corresponding argument value Duration: construction execution
Exception-handling parameter	variables in a catch clause	a new exception-handling parameter is created each time an exception is caught by a catch clause initialized with the actual object associated with the exception Duration: catch clause execution
Local variables	declared by local variable declarations	a new local variable is created whenever flow of control enters a new block or for statement initialized to whatever value is explicitly set within the block or for statement Duration: execution of the block or for statement

Tips

only field variables are automatically initialized to their types default value; local variables must be explicitly initialized
arrays are initialized to the default value of their type when they are created, not declared, even if they are local variables

Traps

an empty string vs null as the default value for a String object

Source	Package	Import	Class	Interface	Constructors
Methods	main()	Identifiers	Keywords	Defaults	Arrays
Primitives	# Literals	char Literal	String Literals	Class Literals