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Java cheat sheet
Educative Answers TeamThe following Java cheat sheet will help review the basic syntax of the language.
Main function
The entry point of a Java program is the main() function. It has the following signature:
public static void main(String args[]) {
// Code goes here...
}
Primitive data types
class HelloWorld {
public static void main(String args[]) {
byte b = 1;
System.out.println("Byte: " + b);
short s = 50;
System.out.println("short: " + s);
int i = 255;
System.out.println("Integer: " + i);
long l = 1500;
System.out.println("Long: " + l);
float f = 250.6f;
System.out.println("Float: " + f);
double d = 134567.89;
System.out.println("Double: " + d);
char c = 'k';
System.out.println("Char: " + c);
boolean flag = false;
System.out.println("Boolean: " + flag);
}
}| Type | Minimum Value | Maximum Value |
|---|---|---|
byte |
||
short |
||
int |
||
long |
Non-primitive data types
1. String
String is the most commonly used data type in Java. Some frequently used methods of this class are demonstrated below:
class HelloWorld {
public static void main( String args[] ) {
String str = new String(" Hello World! ");
// Remove leading and trailing whitespaces:
str = str.trim();
// Get index of a character:
int i = str.indexOf('W');
// Get length of string:
int len = str.length();
// Replace each occurrence of a character:
String s = str.replace("!", " :)");
// Convert to lowercase:
s = s.toLowerCase(); // Opposite to toUpperCase()
System.out.println("Original: " + str +
"\nLength: " + len +
"\nChanged: " + s);
}
}2. Arrays
Arrays can be defined and referenced using the following syntax:
In Java, Arrays can be easily converted into a
Stringusing the statictoString()method in theArraysclass.
import java.util.*;
class HelloWorld {
public static void main( String args[] ) {
// Declare an int array of size 10:
int[] seq = new int[10];
// Populate the array:
for(int i = 0; i < seq.length; ++i)
seq[i] = i;
// Print the array:
System.out.println("int array: " + Arrays.toString(seq));
// Declare and fill a char array:
char[] s = {'a', 'b', 'c', 'd'};
// Print the char array:
System.out.println(s);
}
}Conditional statements
1. If-else
class HelloWorld {
public static void main(String args[]) {
int i = 0;
if(i < 0)
System.out.println("Negative int");
else if (i > 0)
System.out.println("Positive int");
else
System.out.println("int = 0");
}
}2. Switch statements
class HelloWorld {
public static void main(String args[]) {
int i = 0;
switch(i){
case 0:
System.out.println("int = 0");
break;
default:
System.out.println("int != 0");
}
}
}Classes and interface
// Interface:
interface Workable{
public void powerOn();
public void powerOff();
}
// Abstract class:
abstract class Machine implements Workable{
private boolean state;
protected String name;
// Constructor:
Machine(){
state = false;
setName();
}
// Abstract class:
public abstract void setName();
// Implemented interface's functions:
@Override
public void powerOn(){
System.out.println("Powering on " + name);
state = true;
}
@Override
public void powerOff(){
System.out.println("Powering off " + name);
state = false;
}
}
// Concrete class:
class Computer extends Machine{
@Override
public void setName(){
name = "PC";
}
// Entry point:
public static void main(String args[]) {
Computer pc = new Computer();
pc.powerOn();
pc.powerOff();
}
}Loops
class HelloWorld {
public static void main( String args[] ) {
// while loop:
System.out.print("while loop: ");
int i = 0;
while(i < 10){
System.out.print(i + " ");
++i;
}
// for loop:
System.out.print("\nfor loop: ");
for(; i < 20; ++i)
System.out.print(i + " ");
// for-each loop:
System.out.print("\nfor-each loop: ");
char[] arr = {'s','e','n','t','e','n','c','e'};
for(char c: arr)
System.out.print(c);
}
}RELATED TAGS
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