Predefined methods (course memo + handout)

Java's built-in utility classes/methods for strings, numbers, randomness, and the Object base class.

String methods

length(), toLowerCase(), toUpperCase(), substring(start, end), isEmpty(), equals(String), equalsIgnoreCase(String), trim(), startsWith(String), contains(String), endsWith(String), replaceAll(target, replacement), split(separator).

Method	Description	Example	Output
length()	length of the string	"Sarah".length()	5
isEmpty()	whether the string is empty	"".isEmpty()	true
equals(s)	equal to s, case-sensitive	"blue".equals("Blue")	false
equalsIgnoreCase(s)	equal to s, case-insensitive	"blue".equalsIgnoreCase("Blue")	true
startsWith(s)	starts exactly with s	"hello".startsWith("he")	true
contains(s)	contains s	"hello".contains("el")	true
endsWith(s)	ends exactly with s	"hello".endsWith("lo")	true
toUpperCase()	new copy, all uppercase	"Hello".toUpperCase()	"HELLO"
toLowerCase()	new copy, all lowercase	"Hello".toLowerCase()	"hello"
trim()	new copy without leading/trailing spaces	" hi ".trim()	"hi"
substring(start, end)	new string from start to end - 1	"Hey there!".substring(1,3)	"ey"
replaceAll(target, repl)	new copy with all target replaced	"the car is on the street".replaceAll("the","that")	"that car is on that street"
split(sep)	String[] of pieces cut on sep (wrap in List.of(...) for a List)	List.of("eggs, tomatoes".split(", "))	["eggs", "tomatoes"]

All of these return a new string/value — strings are immutable, none of these mutate the original.

Randomization (java.util.Random)

import java.util.Random;

public class Randomization {
    public void example() {
        Random random = new Random();

        int number = random.nextInt();          // any possible int
        int numberBound = random.nextInt(4);     // 0 to (4 - 1)
        int min = 1, max = 6;
        int diceRoll = min + random.nextInt((max - min) + 1); // dice-roll pattern, min..max inclusive
        double zeroToOne = random.nextDouble();  // 0.0 to 1.0
        boolean trueOrFalse = random.nextBoolean();
    }
}

Create one Random instance and reuse it for as many calls as needed — no need to recreate it each time.

Method	Description
nextInt()	random int across the full int range
nextInt(limit)	random int in [0, limit - 1]
nextDouble()	random double in [0.0, 1.0)
nextBoolean()	random true/false

Ranged pattern (min to max inclusive): min + random.nextInt((max - min) + 1).

Object methods

Every class in Java — including custom ones — is ultimately an Object, so every class inherits these methods (and can override them):

Method	Description	Example	Output
toString()	string form of the object; default is the class name + memory address unless overridden	Integer.valueOf(5).toString()	"5" (built-ins return sensible values; custom classes need an override, see equals/hashCode/toString)
equals(other)	equal to other; default compares memory addresses (identity) unless overridden	Integer.valueOf(5).equals(2)	false
compareTo(other)	0 if equal, negative if smaller, positive if bigger (numbers compare by value, strings alphabetically, other objects by default via memory address)	Integer.valueOf(5).compareTo(2)	1

Because Java can always ask for an Object and get any class back, this is also the basis for how generic utility code (collections, equals, etc.) can work across arbitrary types.