Java Quick Reference

Common Java classes and methods for AP Computer Science A

📝 Reference information commonly used in AP CS A exam preparation

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String8 methods

String(String str)

Constructs a new String object that represents the same sequence of characters as str

Example:

String name = new String("Hello");

int length()

Returns the number of characters in a String object

Example:

String text = "Hello World";
int len = text.length(); // Returns 11

String substring(int from, int to)

Returns the substring beginning at index from and ending at index to – 1

Example:

String text = "Hello World";
String sub = text.substring(0, 5); // Returns "Hello"

String substring(int from)

Returns substring(from, length())

Example:

String text = "Hello World";
String sub = text.substring(6); // Returns "World"

int indexOf(String str)

Returns the index of the first occurrence of str; returns -1 if not found

Example:

String text = "Hello World";
int pos = text.indexOf("World"); // Returns 6

boolean equals(Object other)

Returns true if this corresponds to the same sequence of characters as other; returns false otherwise

Example:

String str1 = "Hello";
String str2 = "Hello";
boolean same = str1.equals(str2); // Returns true

int compareTo(String other)

Returns a value < 0 if this is less than other; returns zero if this is equal to other; returns a value > 0 if this is greater than other. Strings are ordered based upon the alphabet.

Example:

String str1 = "apple";
String str2 = "banana";
int result = str1.compareTo(str2); // Returns negative value

String[] split(String del)

Returns a String array where each element is a substring of this String, which has been split around matches of the given expression del

Example:

String text = "apple,banana,cherry";
String[] fruits = text.split(","); // Returns ["apple", "banana", "cherry"]

Integer3 methods

Integer.MIN_VALUE

The minimum value represented by an int or Integer

Example:

int min = Integer.MIN_VALUE; // -2147483648

Integer.MAX_VALUE

The maximum value represented by an int or Integer

Example:

int max = Integer.MAX_VALUE; // 2147483647

static int parseInt(String s)

Returns the String argument as an int

Example:

String str = "123";
int num = Integer.parseInt(str); // Returns 123

Double1 method

static double parseDouble(String s)

Returns the String argument as a double

Example:

String str = "3.14";
double num = Double.parseDouble(str); // Returns 3.14

Math5 methods

static int abs(int x)

Returns the absolute value of an int value

Example:

int result = Math.abs(-5); // Returns 5

static double abs(double x)

Returns the absolute value of a double value

Example:

double result = Math.abs(-3.14); // Returns 3.14

static double pow(double base, double exponent)

Returns the value of the first parameter raised to the power of the second parameter

Example:

double result = Math.pow(2, 3); // Returns 8.0

static double sqrt(double x)

Returns the nonnegative square root of a double value

Example:

double result = Math.sqrt(16); // Returns 4.0

static double random()

Returns a double value greater than or equal to 0.0 and less than 1.0

Example:

double random = Math.random(); // Returns 0.0 to 0.999...

ArrayList6 methods

int size()

Returns the number of elements in the list

Example:

ArrayList<String> list = new ArrayList<>();
list.add("item");
int size = list.size(); // Returns 1

boolean add(E obj)

Appends obj to end of list; returns true

Example:

ArrayList<String> list = new ArrayList<>();
boolean added = list.add("Hello"); // Returns true

void add(int index, E obj)

Inserts obj at position index (0 <= index <= size), moving elements at position index and higher to the right (adds 1 to their indices) and adds 1 to size

Example:

ArrayList<String> list = new ArrayList<>();
list.add(0, "First"); // Inserts at index 0

E get(int index)

Returns the element at position index in the list

Example:

ArrayList<String> list = new ArrayList<>();
list.add("Hello");
String item = list.get(0); // Returns "Hello"

E set(int index, E obj)

Replaces the element at position index with obj; returns the element formerly at position index

Example:

ArrayList<String> list = new ArrayList<>();
list.add("Hello");
String old = list.set(0, "Hi"); // Returns "Hello"

E remove(int index)

Removes element from position index, moving elements at position index + 1 and higher to the left (subtracts 1 from their indices) and subtracts 1 from size; returns the element formerly at position index

Example:

ArrayList<String> list = new ArrayList<>();
list.add("Hello");
String removed = list.remove(0); // Returns "Hello"

File1 method

File(String pathname)

The File constructor that accepts a String pathname

Example:

// Example not available

Scanner8 methods

Scanner(File f)

The Scanner constructor that accepts a File for reading

Example:

File file = new File("data.txt");
Scanner scanner = new Scanner(file);

int nextInt()

Returns the next int read from the file or input source if available. If the next int does not exist or is out of range, it will result in an InputMismatchException.

Example:

Scanner scanner = new Scanner(System.in);
int num = scanner.nextInt(); // Reads next integer

double nextDouble()

Returns the next double read from the file or input source. If the next double does not exist, it will result in an InputMismatchException.

Example:

Scanner scanner = new Scanner(System.in);
double num = scanner.nextDouble(); // Reads next double

boolean nextBoolean()

Returns the next boolean read from the file or input source. If the next boolean does not exist, it will result in an InputMismatchException.

Example:

Scanner scanner = new Scanner(System.in);
boolean flag = scanner.nextBoolean(); // Reads next boolean

String nextLine()

Returns the next line of text as a String read from the file or input source; can return the empty string if called immediately after another Scanner method that is reading from the file or input source

Example:

Scanner scanner = new Scanner(System.in);
String line = scanner.nextLine(); // Reads entire line

String next()

Returns the next String read from the file or input source

Example:

Scanner scanner = new Scanner(System.in);
String word = scanner.next(); // Reads next word

boolean hasNext()

Returns true if there is a next item to read in the file or input source; false otherwise

Example:

Scanner scanner = new Scanner(System.in);
if (scanner.hasNext()) {
    String input = scanner.next();
}

void close()

Closes this scanner

Example:

Scanner scanner = new Scanner(System.in);
scanner.close(); // Closes the scanner

Object2 methods

boolean equals(Object other)

Returns true if this object is equal to other; false otherwise

Example:

String str1 = "Hello";
String str2 = "Hello";
boolean same = str1.equals(str2); // Returns true

String toString()

Returns a string representation of this object

Example:

Integer num = 42;
String str = num.toString(); // Returns "42"

Java Quick Reference

Quick Jump

String8 methods

Integer3 methods

Double1 method

Math5 methods

ArrayList6 methods

File1 method

Scanner8 methods

Object2 methods

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