Java cheatsheet

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

POLYMORPHISM:
Single Inheritance with “extends”
class A{ }
class B extends A{ }
abstract class C { }
class D extends C { }
class E extends D
Abstract methods
abstract class F {
abstract int bla();
}
class G extends F {
int bla() { //required method
return 5;
}
}
Multiple Inheritance of interfaces with
“implements” (fields not inherited)
interface H {
void methodA();
boolean methodB(int arg);
}
interface I extends H{
void methodC();
}
interface K {}
class J extends F implements I, K {
int bla() { return 5; } //required from F
void methodA(){} //required from H
boolean methodB(int a) { //req from A
return 1;
}
void methodC(){} //required from I
}
Type inference:
A x = new B(); //OK
B y = new A(); //Not OK
C z = new C(); //Cannot instantiate abstract
//Method calls care about right hand type
(the instantiated object)
//Compiler checks depend on left hand type
GENERICS:
class MyClass<T> {
T value;
T getValue() { return value; }
}
class ExampleTwo<A,B> {
A x;
B y;
}
class ExampleThree<A extends List<B>,B> {
A list;
B head;
}
//Note the extends keyword here applies as
well to interfaces, so A can be an interface
that extends List<B>
JAVA COLLECTIONS:
List<T>: Similar to arrays
ArrayList<T>: Slow insert into middle
//ArrayList has fast random access
LinkedList<T>: slow random access
//LinkedList fast as queue/stack
Stack: Removes and adds from end
List Usage:
boolean add(T e);
void clear(); //empties
boolean contains(Object o);
T get(int index);
T remove(int index);
boolean remove(Object o);
//remove uses comparator
T set(int index, E val);
Int size();
List Traversal:
for(int i=0i<x.size();i++) {
//use x.get(i);
}
//Assuming List<T>:
for(T e : x) {
//use e
}
Queue<T>: Remove end, Insert beginning
LinkedList implements Queue
Queue Usage:
T element(); // does not remove
boolean offer(T o); //adds
T peek(); //pike element
T poll(); //removes
T remove(); //like poll
Traversal: for(T e : x) {}
Set<T>: uses Comparable<T> for uniqueness
TreeSet<T>, items are sorted
HashSet<T>, not sorted, no order
LinkedHashSet<T>, ordered by insert
Usage like list: add, remove, size
Traversal: for(T e : x) {}
Map<K,V>: Pairs where keys are unique
HashMap<K,V>, no order
LinkedHashMap<K,V> ordered by insert
TreeMap<K,V> sorted by keys
V get(K key);
Set<K> keySet(); //set of keys
V put(K key, V value);
V remove(K key);
Int size();
Collection<V> values(); //all values
Traversal: for-each w/ keyset/values
java.util.PriorityQueue<T>
A queue that is always automatically sorted
using the comparable function of an object
public static void main(String[] args) {
Comparator<String> cmp= new LenCmp();
PriorityQueue<String> queue =
new PriorityQueue<String>(10, cmp);
queue.add("short");
queue.add("very long indeed");
queue.add("medium");
while (queue.size() != 0)
System.out.println(queue.remove());
}
class LenCmp implements Comparator<String> {
public int compare(String x, String y){
return x.length() – y.length();
}
}
java.util.Collections algorithms
Sort Example:
//Assuming List<T> x
Collections.sort(x); //sorts with comparator
Sort Using Comparator:
Collections.sort(x, new Comparator<T>{
public int compareTo(T a, T b) {
//calculate which is first
//return -1, 0, or 1 for order:
return someint;
}
}
Example of two dimensional array sort:
public static void main(final String[] a){
final String[][] data = new String[][] {
new String[] { "20090725", "A" },
new String[] { "20090726", "B" },
new String[] { "20090727", "C" },
new String[] { "20090728", "D" } };
Arrays.sort(data,
new Comparator<String[]>() {
public int compare(final String[]
entry1, final String[] entry2) {
final String time1 = entry1[0];
final String time2 = entry2[0];
return time1.compareTo(time2);
}
});
for (final String[] s : data) {
System.out.println(s[0]+" "+s[1]);
}
}
}
More collections static methods:
Collections.max( … ); //returns maximum
Collections.min( … ); //returns maximum
Collections.copy( A, B); //A list into B
Collections.reverse( A ); //if A is list
® 2013 Kapparate, © 2013 Kapparate Operating LLC

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