Java basics

Archived

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Sources:

Java All-In One Desk Reference For Dummies, Second Edition

Naming conventions

Variables/methods

Camel case where first word is uncapitalized:

myMethod

Classes

Camel case where first word is capitalized:

MyClass

Constants (variables declared using the final keyword)

Use all caps:

final int PI = 3.14;

Primitive types

Notes

• not objects

Primitive wrapper objects

| int | Integer | | short | Short | | long | Long | | byte | Byte | | float | Float | | double | Double | | char | Character | | boolean | Boolean |

since Java 1.5, conversion to and from wrapper objects is automatic

Parsing objects to primitive types

int x = Integer.parseInt("100");

Variables

Declare a constant

(variable that can only be assigned once)

final TYPE CONSTANTNAME;

Initializers

• Block of code that initializes variables
• Don't use these, but you should be able to recognize them:

public int myNumber;

{
    // code to initialize myNumber
}

Strings

Notes

• When doing a lot of string manipulation, use StringBuilder
  • If you're using threads, use StringBuffer
• String, StringBuilder, and StringBuffer all implement CharSequence interface

Lists

Notes

• use List (http://download.oracle.com/javase/tutorial/collections/interfaces/list.html)
  • See below
• If you'll be doing a lot of modification to your list/array (insertions, deletions, additions), use a LinkedList

Lists

List<String> myList = new ArrayList<String>();
myList.add(someString);
myList.get(int index);
myList.remove(int index);
myList.size();
myList.isEmpty();

Get the last element of a list

myList.get(myList.size() - 1);

Nested Lists

List<ArrayList<String>> listOlists = new ArrayList<ArrayList<String>>();

Nested Arrays

int scores[][] = new int[5][2];

scores[0][1] = 93;
scores[0][2] = 74;

Associative arrays

Use Map (http://download.oracle.com/javase/tutorial/collections/interfaces/map.html)

Map<String, Integer> m = new HashMap<String, Integer>();
m.put("key", value);
m.get("key");
m.isEmpty();
m.containsKey("key");

Simpler usage:

Map map = new HashMap();

Iterate over a Map

• Java 8+:

  nameCounts.forEach((key, value) -> {
      doSomething(key, value);
  });

• < Java 8: Iterate through a HashMap

Sort a Map by value

Sort a Map<Key, Value> by values

Loops

For loop

for (int i=1; i<11; i++) {
    System.out.println("Count is: " + i);
}

(counts from 1 to 10)

Enhanced for loop

for (type identifier : array) {
}

String[] things = { "first thing", "second thing", "third thing" };
for (String thing : things) {
    System.out.println(thing);
}

Conditionals

Format of if statements

if ( EXPRESSION ) {
    doSomething();
} else if ( EXPRESSION ) {
    doSomethingElse();
}

Note: the parentheses after the if statement are required.

Access

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

Exceptions

Catch the Exception class to get all exceptions

try {
    doSomething();
} catch (Exception e) {
}

Useful methods of exceptions

| String getMessage() | A text message that describes the error | | void PrintStackTrace() | Prints stack trace to stderr | | String toString()| Returns a description of the exception which includes the name of the exception class followed by a colon and the getMessage message |