THE JAVA LANGUAGE CHEAT SHEET
Primitive Types:
INTEGER: byte(8bit),short(16bit),int(32bit),
long(64bit),DECIM:float(32bit),double(64bit)
,OTHER: boolean(1bit), char (Unicode)
HEX:0x1AF,BINARY:0b00101,LONG:8888888888888L
CHAR EXAMPLES: ‘a’,’\n’,’\t’,’\’’,’\\’,’\”’
Primitive Operators
Assignment Operator: = (ex: int a=5,b=3; )
Binary Operators (two arguments): + - * / %
Unary Operators: + - ++ --
Boolean Not Operator (Unary): !
Boolean Binary: == != > >= < <=
Boolean Binary Only: && ||
Bitwise Operators: ~ & ^ | << >> >>>
Ternary Operator: bool?valtrue:valfalse;
Casting, Conversion
int x = (int)5.5; //works for numeric types
int x = Integer.parseInt(“123”);
float y = Float.parseFloat(“1.5”);
int x = Integer.parseInt(“7A”,16); //fromHex
String hex = Integer.toString(99,16);//toHex
//Previous lines work w/ binary, other bases
java.util.Scanner, input, output
Scanner sc = new Scanner(System.in);
int i = sc.nextInt(); //stops at whitespace
String line = sc.nextLine(); //whole line
System.out.println(“bla”); //stdout
System.err.print(“bla”); //stderr,no newline
java.lang.Number types
Integer x = 5; double y = x.doubleValue();
double y = (double)x.intValue();
//Many other methods for Long, Double, etc
java.lang.String Methods
//Operator +, e.g. “fat”+”cat” -> “fatcat”
boolean equals(String other);
int length();
char charAt(int i);
String substring(int i, int j); //j not incl
boolean contains(String sub);
boolean startsWith(String pre);
boolean endsWith(String post);
int indexOf(String p); //-1 if not found
int indexOf(String p, int i); //start at i
int compareTo(String t);
//“a”.compareTo(“b”) -> -1
String replaceAll(String str, String find);
String[] split(String delim);
StringBuffer, StringBuilder
StringBuffer is synchronized StringBuilder
(Use StringBuilder unless multithreaded)
Use the .apend( xyz ) methods to concat
toString() converts back to String
java.lang.Math
Math.abs(NUM),Math.ceil(NUM),Math.floor(NUM)
,Math.log(NUM),Math.max(A,B),Math.min(C,D),
Math.pow(A,B),Math.round(A),Math.random()
IF STATEMENTS:
if( boolean_value ) { STATEMENTS }
else if( bool ) { STATEMENTS }
else if( ..etc ) { STATEMENTS }
else { STATEMENTS }
//curly brackets optional if one line
LOOPS:
while( bool ) { STATEMENTS }
for(INIT;BOOL;UPDATE) { STATEMENTS }
//1INIT 2BOOL 3STATEMENTS 4UPDATE 5->Step2
do{ STATEMENTS }while( bool );
//do loops run at least once before checking
break; //ends enclosing loop (exit loop)
continue; //jumps to bottom of loop
ARRAYS:
int[] x = new int[10]; //ten zeros
int[][] x = new int[5][5]; //5 by 5 matrix
int[] x = {1,2,3,4};
x.length; //int expression length of array
int[][] x = {{1,2},{3,4,5}}; //ragged array
String[] y = new String[10]; //10 nulls
//Note that object types are null by default
//loop through array:
for(int i=0;i<arrayname.length;i++) {
//use arrayname[i];
}
//for-each loop through array
int[] x = {10,20,30,40};
for(int v : x) {
//v cycles between 10,20,30,40
}
//Loop through ragged arrays:
for(int i=0;i<x.length;i++)
for(int j=0;j<x[i].length;j++) {
//CODE HERE
}
//Note, multi-dim arrays can have nulls
//in many places, especially object arrays:
Integer[][] x = {{1,2},{3,null},null};
FUNCTIONS / METHODS:
Static Declarations:
public static int functionname( … )
private static double functionname( … )
static void functionname( … )
Instance Declarations:
public void functionname( … )
private int functionname( … )
Arguments, Return Statement:
int myfunc(int arg0, String arg1) {
return 5; //type matches int myfunc
}
//Non-void methods must return before ending
//Recursive functions should have an if
//statement base-case that returns at once
CLASS/OBJECT TYPES:
INSTANTIATION:
public class Ball {//only 1 public per file
//STATIC FIELDS/METHODS
private static int numBalls = 0;
public static int getNumBalls() {
return numBalls;
}
public static final int BALLRADIUS = 5;
//INSTANCE FIELDS
private int x, y, vx, vy;
public boolean randomPos = false;
//CONSTRUCTORS
public Ball(int x, int y, int vx, int vy)
{
this.x = x;
this.y = y;
this.vx = vx;
this.vy = vy;
numBalls++;
}
Ball() {
x = Math.random()*100;
y = Math.random()*200;
randomPos = true;
}
//INSTANCE METHODS
public int getX(){ return x; }
public int getY(){ return y; }
public int getVX(){ return vx; }
public int getVY(){ return vy; }
public void move(){ x+=vx; y+=vy; }
public boolean touching(Ball other) {
float dx = x-other.x;
float dy = y-other.y;
float rr = BALLRADIUS;
return Math.sqrt(dx*dx+dy*dy)<rr;
}
}
//Example Usage:
public static void main(String[] args) {
Ball x = new Ball(5,10,2,2);
Ball y = new Ball();
List<Ball> balls = new ArrayList<Ball>();
balls.add(x); balls.add(y);
for(Ball b : balls) {
for(Ball o : balls) {
if(b != o) { //compares references
boolean touch = b.touching(o);
}
}
}
}
® 2013 Kapparate, © 2013 Kapparate Operating LLC
