Unit 2 AP Comp Sci A

What is the class and object of:

String tree = new String(“oak”);

Class: String

Object: tree

Null in Java

Represents an object or class data type that has not been created.

Two basic types of data types in Java

Primitive types and class (reference) types.

Primitive type

Holds only one piece of data at a time (e.g. int,double,boolean)

Class (reference) type

Can hold more than one piece of data at a time and is a blueprint that defines the properties and methods that objects created from it will have.

How primitive data types are stored in memory

As the actual value (e.g., int number 13, stores 13)

How class data types are stored in memory

As a memory address pointing to where the actual value is stored (e.g., String a = "Howdy";).

No reference set for a String

Holds the value null

Strings are immutable

They cannot be changed once created. If you modify a string, a new one is created instead of altering the original.

What would be the output of this code?

String alpha = "Toy Story"; 

String beta = alpha; 

alpha = alpha + " 2"; 

System.out.println(alpha); 

System.out.println(beta);

Toy Story 2

Toy Story

Three ways to create Strings in Java

String alpha = "Wonder Woman";`  

String beta;

beta = "Superman";

String gamma = new String("Spiderman");

+ operator in Java

Concatenates strings and adds integers.

Concatenation with strings

"Glues" two strings together (e.g.

+= operator with strings

combines the two strings into one

Controlling order of operations with strings and integers

By using parentheses

Escape sequences in Java

\" - double quote  

\n - new line  

\t - horizontal tab  

\\ - backslash  

\' - single quote 

Characters stored in a string

Each character is stored with an index starting at 0.

.length() method

Returns the number of characters in the string.

String s = “charizard”;

String t = “charizard”;

s.equals(t)

Tests if two strings are equal (returns true or false).

.substring(start,stop)

gets a part of a string from the start index to the stop index (not including the stop)

.toUpperCase() and .toLowerCase() methods

Convert all letters to uppercase or lowercase

.compareTo(other)

Compares two strings alphabetically.

.indexOf(str)

Searches for a character or string and returns its index position.

Class in Java

"Recipe" for creating objects.

Object in Java

Specific instance of a class.

Example of creating a String object

String tree = new String("oak")

Keyword new in Java

Allocates memory for the new object.

Assigning one object to another

Both variables now point to the same memory location.