Tip 40

You should be able to identify how many objects are on the heap after a code statement
or block executes.

Thread[] threads = new Thread[10];

When the above code is executed , there will only be one object in the heap. That is a single array object that holds ten thread references. The thread objects are neither created or assigned to these reference variables.

Language Fundamentals - Arrays

An article on arrays in Java.

Tip 39

Characters are 16-bit unsigned integers.You can assign a number literal, assuming it will fit into the unsigned 16-bit range.

These are legal

char ex1 = 0x892;

char ex2 = 982;

char ex3 = (char) 70000;

char ex4 = (char) -98;

char ex5 = '\"';

char ex6 = '\n';

char ex7 = '\u004E';

char ex8 = 'a';

char ex9 = '@';


And the following are illegal

char ex1 = -29; // needs a cast

char ex2 = 70000 // needs a cast

Tip 38

Local variables should be initialized. Otherwise the compiler will complain. Instance variables on the other hand will be initialized to the default values of their variable types automatically.

Tip 37

Array elements are always initialized with default values, regardless of where they are declared.

Storing Information with Java Arrays

A free tutorial on Java arrays.

Tip 36

A collection of some salient points of the Java programming language.

Points to remember

Tip 35

If a class implements an interface the interface is considered a super type.

Tip 34

Interfaces support multiple inheritance and can extend many interfaces.

Tip 33

Interfaces are by default public and abstract. So are the methods in them. The variables in interfaces are by default public static and final.

Tip 32

All possible combinations of features and modifiers

Modifier	Class	Variable	Method	Constructor	Free-Floating Block
public	yes	yes	yes	yes	no
protected	no	yes	yes	yes	no
default	yes	yes	yes	yes	yes
private	no	yes	yes	yes	no
final	yes	yes	yes	no	no
abstract	yes	no	yes	no	no
static	no	yes	yes	no	yes
native	no	no	yes	no	no
transient	no	yes	no	no	no
volatile	no	yes	no	no	no
synchronized	no	no	yes	no	yes
strictfp	yes	no	yes	no	no

For further information refer the following

Java Modifier Summary
A good summarized explanation on Java Modifiers.

Tip 31

You cannot change the value of a transient variable.

For further information on transient variables refer the following

Declaring a Transient Variable
A brief explanation on how to use transient variables.

Tip 30

Statics will be loaded at class load time.

Tip 29

Static method member variables are not allowed.

class inte {

public static void main(String[] args) {

static int b=3; <---------- Compiler Error


}
}

Tip 28

The abstract key word cannot be used with the following in methods

- final
- static
- private
- strictfp
- syncronized

The abstract key word cannot be used with the following in classes

- final

Tip 27

Abstract methods should be overridden at least or before the first concrete class.

If there is an abstract method in a class, the class should be an Abstract class.

Tip 26

Cannot modify static methods with abstract.

Tip 25

Final methods cannot be overridden.

Tip 24

You cannot extend final classes.

Tip 23

You cannot use any access modifiers inside the main method.

Tip 22

One source file can only have one public class.

Tip 21

Member variables in a class are implicitly initialized to its default values. Method local variables are not.

Tip 20

An inbuilt cast applies on Assignment operators (+= , -= ...)

Tip 19

Division by 0 results in an arithmetic exception for integers

Division by 0 results in negative or positive infinity values for floating point numbers.

Tip 18

Precedence levels of arithmetic operators

* Multiplication

% Modulous

/ Division

+ Addition

- Substraction


All the arithmetic operators ( + , - , *, %, /) are subjected to arithmetic promotions.

For further information on operators refer the following

Operators and Assignments
This article gives a detailed explanation on operators.

Tip 17

Arithmetic promotions on unary operators

+ -----------> Arithmetic promotions apply

- -----------> Arithmetic promotions apply

++ -----------> Arithmetic promotions don't apply

-- -----------> Arithmetic promotions don't apply

~ -----------> Arithmetic promotions apply

! -----------> Arithmetic promotions don't apply

Tip 16

Operator precedence levels are as follows

Unary	++ -- + - ! ~ ()
Arithmetic	* / % + -
Shift	<< >> >>>
Comparison	== !=
Bitwise	& ^ |
Short-circuit	&& ||
Ternary	?:
Assignment	= "op="

For further information on operators refer the following

Operators and Assignments
This article gives a detailed explanation on operators.

Tip 15

Many operations are subjected to arithmetic promotions.

byte + byte -------> int

byte + short ------> int

short + short ------> int

char+ char --------> int

int + int -----------> int

int + long ---------> long

long + float -------> float

float + double-----> double

double + byte -----> double

Tip 14

It is not legal to give the array size of an array during declaration.

Eg: int arr [9]; // Illegal

The JVM does not allocate space to the array till you initialize it.

The follwing articles provide further information.

Java Arrays
A good introduction on arrays.

Tip 13

Some questions may have numbers where booleans are required. This type of code generates compiler errors.

For eg:
int x = 1;

if (x)
{

System.out.println("YES");

}

Tip 12

Misleading numeric literals can be given.

For e.g Numbers separated by commas
int x = 35,897

Tip 11

All integers are implicitly int literals.

e.g: byte num = 10; <-- int literal within the range of byte

All floating point numbers are implicitly double literals.

e.g float num = 15.9 <-- Compiler Error : Possible loss of precision

float num = 15.9f <-- Compiles

Tip 10

Octal and Hexadecimal literals are not case sensitive. An octal number can have upto 21 digits (excluding the leading 0) and a hexadecimal number can have upto 16 digits(excluding the leading 0x).

For eg: int num = 0x5 and int num = 0X5 are both legal

Tip 9

Integer numbers can be represented in 3 ways.

• decimal ( 10 base)
• octal ( 8 base)
• hexadecimal (16 base)

The following articles provide further information.

A Practical Guide to Binary, Decimal and Hexadecimal Numbers
A brief explanation on the Binary, Decimal and Hexadecimal numbering systems.

Number System
A good explanation on number systems with examples.

Octal Number System
A good article on octal number systems.

A Brief Explanation of Hexadecimal Numbers
A nice tutorial on hexadecimal numbers.

Hexadecimal Number System
A comprehensive explanation on hexadecimal numbers.

Tip 8

Char is an integral data type. It can be assigned to any number type which is large enough to hold 65535 (2¹⁶-1)

The following articles provide further information.

Primitive Data Types
This article provides a comprehensive explanation about the primitive data types in Java.

Tip 7

All number types in Java are signed and have the following ranges.

byte 8 Bits (-2⁷ to 2⁷-1)
short 16 Bits (-2¹⁵ to 2¹⁵-1)
int 32 Bits (-2³¹ to 2³¹-1)
long 64 Bits (-2⁶³ to 2⁶³-1)
float 32 Bits
double 64 Bits

The following articles provide further information.

Primitive Data Types
This article provides a comprehensive explanation about the primitive data types in Java.

Tip 6

Words similar to the actual keyword with minor differences will be given.
For eg: extend in place of extends

Tip 5

Null , False and True are actually Literals not keywords. But you cannot create identifiers with these words either.
You will not be asked to differentiate between keywords and reserved literals (true, false, null).

Tip 4

There can be questions that use a keyword as the name of a method or variable.
For eg: Questions asking about logic problems in a piece of code, but the real problem will be that the code won't even compile because of the illegal use of the keyword.

Tip 3

This is an easy way to remember the 52 Keywords.

Reserved - const , goto (2)

Error Handling - try, catch, finally, throw, throws, assert (6)

Modifiers - private, public, protected, final, static, abstract, native, synchronized,
transcient , scritfp, volatile ( 11)

Constructs - if, else, while , for , do, return, switch, case, default, continue, break (11)

Other - class, package, implements, extends, import, interface, super, this, new, instanceof (10)

Literals - true, false, null (3)

Data & return types - int, short, byte, char, long, boolean, double, float, void (9)

The following articles provide further information

Java Language Keywords
This article lists all the Java keywords available.

PDF File - Java Language Keywords
This pdf file gives a comprehensive explanation about all the keywords including code examples.