Java Core Packages

Java

Java Basics Java Core Packages Java Collections Java Multithreading OOP Useful Libraries Spring Boot REST API w/ Spring Boot Login / Authorization

java.lang

Core language classes

String
Math
Object
Integer
Thread

java.util

Data structures, algorithms, and utilities classes

ArrayLists
Collections
Scanner

java.io

Reading/writing files and streams

File
FileReader
BufferedWriter
ObjectOutputStream

javax.swing

Building desktop GUI applications

JFrame
JButton
JLabel
JTextField

String

Return Type: Boolean

Method	Description	Example	Result
`equals(Object anObject)`	Checks if this string is equal to another object.	`"hello".equals("hello")`	`true`
`equalsIgnoreCase(String anotherString)`	Checks equality ignoring case.	`"Hello".equalsIgnoreCase("hello")`	`true`
`contains(CharSequence s)`	Returns true if string contains the specified sequence.	`"hello".contains("ell")`	`true`
`startsWith(String prefix)`	Checks if string starts with specified prefix.	`"hello".startsWith("he")`	`true`
`startsWith(String prefix, int offset)`	Checks if string starts with prefix at the given offset.	`"hello".startsWith("ll", 2)`	`true`
`endsWith(String suffix)`	Checks if string ends with specified suffix.	`"hello".endsWith("lo")`	`true`
`isEmpty()`	Returns true if string length is 0.	`"".isEmpty()`	`true`
`matches(String regex)`	Returns true if string matches regex.	`"abc123".matches("\\w+\\d+")`	`true`

Return Type: int

Method	Description	Example	Result
`length()`	Returns number of characters in the string.	`"hello".length()`	`5`
`indexOf(int ch)`	Returns index of first occurrence of character (-1 if not found).	`"hello".indexOf('l')`	`2`
`indexOf(String str)`	Returns index of first occurrence of substring (-1 if not found).	`"hello".indexOf("ll")`	`2`
`lastIndexOf(int ch)`	Returns index of last occurrence of character (-1 if not found).	`"hello".lastIndexOf('l')`	`3`
`lastIndexOf(String str)`	Returns index of last occurrence of substring (-1 if not found).	`"hello".lastIndexOf("l")`	`3`
`compareTo(String anotherString)`	Compares two strings lexicographically.	`"abc".compareTo("abd")`	`-1`
`compareToIgnoreCase(String str)`	Compares two strings ignoring case.	`"abc".compareToIgnoreCase("ABC")`	`0`
`codePointAt(int index)`	Returns Unicode code point at specified index.	`"ABC".codePointAt(1)`	`66`
`codePointBefore(int index)`	Returns Unicode code point before the specified index.	`"ABC".codePointBefore(2)`	`66`
`codePointCount(int beginIndex, int endIndex)`	Returns the number of Unicode code points in the specified text range.	`"ABC".codePointCount(0,3)`	`3`

Return Type: char / String / String[]

Method	Description	Example	Result
`charAt(int index)`	Returns character at specified index.	`"hello".charAt(1)`	`'e'`
`substring(int beginIndex)`	Returns substring from beginIndex to end.	`"hello".substring(2)`	`"llo"`
`substring(int beginIndex, int endIndex)`	Returns substring from beginIndex (inclusive) to endIndex (exclusive).	`"hello".substring(1,4)`	`"ell"`
`concat(String str)`	Concatenates specified string to the end.	`"Hello".concat(" World")`	`"Hello World"`
`replace(char oldChar, char newChar)`	Replaces all occurrences of oldChar with newChar.	`"hello".replace('l','p')`	`"heppo"`
`replace(CharSequence target, CharSequence replacement)`	Replaces all occurrences of target sequence with replacement.	`"hi hi".replace("hi","yo")`	`"yo yo"`
`replaceFirst(String regex, String replacement)`	Replaces first substring matching regex with replacement.	`"a1b2c3".replaceFirst("\\d","")`	`"ab2c3"`
`replaceAll(String regex, String replacement)`	Replaces all substrings matching regex with replacement.	`"a1b2c3".replaceAll("\\d","")`	`"abc"`
`trim()`	Removes leading and trailing whitespace.	`" hi ".trim()`	`"hi"`
`toUpperCase()`	Converts string to uppercase.	`"hi".toUpperCase()`	`"HI"`
`toLowerCase()`	Converts string to lowercase.	`"HI".toLowerCase()`	`"hi"`
`split(String regex)`	Splits string around matches of regex.	`"a,b,c".split(",")`	`["a","b","c"]`
`split(String regex, int limit)`	Splits string with limit on number of elements.	`"a,b,c".split(",",2)`	`["a","b,c"]`
`intern()`	Returns canonical representation from string pool.	`"hello".intern()`	`"hello"`
`format(String format, Object... args)`	Returns formatted string.	`String.format("%d + %d = %d",1,2,3)`	`"1 + 2 = 3"`

Return Type: void

Method	Description	Example	Result
`getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)`	Copies characters from string into destination array.	`char[] arr = new char[5]; "hello".getChars(0,5,arr,0)`	—

Object

Return Type: Boolean

Method	Description	Example	Result
`equals(Object obj)`	Indicates whether some other object is "equal to" this one.	`obj1.equals(obj2)`	`true` or `false`
`equalsIgnoreCase` does not exist for Object	—	—	—
`notify()`	Wakes up a single thread waiting on this object's monitor.	`synchronized(obj){ obj.notify(); }`	—
`notifyAll()`	Wakes up all threads waiting on this object's monitor.	`synchronized(obj){ obj.notifyAll(); }`	—

Return Type: int

Method	Description	Example	Result
`hashCode()`	Returns a hash code value for the object.	`obj.hashCode()`	`12345678` (example)
`wait()`	Causes current thread to wait until notified. Returns int? Actually void—so remove from INT table	—	—

Return Type: Object

Method	Description	Example	Result
`clone()`	Creates and returns a copy of this object (if class implements Cloneable).	`obj.clone()`	`new Object()` copy

Return Type: String

Method	Description	Example	Result
`toString()`	Returns a string representation of the object.	`obj.toString()`	`"ClassName@hashcode"`

Return Type: void

Method	Description	Example	Result
`wait()`	Causes current thread to wait until another thread invokes `notify()` or `notifyAll()` on this object.	`synchronized(obj){ obj.wait(); }`	—
`wait(long timeout)`	Waits up to the specified time in milliseconds.	`synchronized(obj){ obj.wait(1000); }`	—
`wait(long timeout, int nanos)`	Waits up to the specified time (ms + nanos).	`synchronized(obj){ obj.wait(1000,500); }`	—

Random

Constructors

Constructor	Description	Example	Notes
`Random()`	Creates a new random number generator using a time-based seed. Each instance will produce different sequences.	`Random rand = new Random();`	Default constructor; uses current time as seed.
`Random(long seed)`	Creates a new random number generator using the given seed. Seeding ensures reproducible results.	`Random rand = new Random(12345L);`	Same seed ⇒ same random sequence.

Return Type: Boolean

Method	Description	Example	Result
`nextBoolean()`	Returns a random boolean value (true or false).	`rand.nextBoolean()`	`true`

Return Type: int

Method	Description	Example	Result
`nextInt()`	Returns a random integer (can be negative).	`rand.nextInt()`	`-123456789`
`nextInt(int bound)`	Returns a random integer between `0` (inclusive) and `bound` (exclusive).	`rand.nextInt(10)`	`7`

Return Type: long

Method	Description	Example	Result
`nextLong()`	Returns a random long value.	`rand.nextLong()`	`-3456789012345L`

Return Type: float

Method	Description	Example	Result
`nextFloat()`	Returns a random float between `0.0` (inclusive) and `1.0` (exclusive).	`rand.nextFloat()`	`0.57293`

Return Type: double

Method	Description	Example	Result
`nextDouble()`	Returns a random double between `0.0` (inclusive) and `1.0` (exclusive).	`rand.nextDouble()`	`0.93527`
`nextGaussian()`	Returns a random value from a standard normal distribution (mean 0, std dev 1).	`rand.nextGaussian()`	`-0.3725`

Return Type: void

Method	Description	Example	Result
`setSeed(long seed)`	Sets the seed for this random number generator, restarting its sequence.	`rand.setSeed(12345L)`	—

Return Type: Stream

Method	Description	Example	Result
`ints()`	Returns an infinite stream of random integers.	`rand.ints().limit(3).forEach(System.out::println)`	`e.g. 12 93 47`
`ints(int streamSize)`	Returns a stream with the specified number of random integers.	`rand.ints(5)`	`[87, 64, 21, 95, 13]`
`ints(int origin, int bound)`	Returns an infinite stream of random integers within the specified range.	`rand.ints(0, 10).limit(3)`	`[2, 7, 9]`
`longs()`	Returns an infinite stream of random long values.	`rand.longs().limit(2)`	`[3487347, -8327493]`
`doubles()`	Returns an infinite stream of random doubles.	`rand.doubles().limit(2)`	`[0.234, 0.894]`