Description

Basics
- Handling Strings
- Collections
Conversions
- Collections - Array
Specifics
- Copy array
- Convert to String

Basics

Handling strings

Selected String methods

Return	Method	Description
String	`.substring(int begin, int end?)`	substring till end-1
boolean	`.contains(charSeq)`	match the sequence
String	`.join(delimiter, elements...)`	joins given string with all string elements in argument using delimiter
String[]	`.split(String regex, int limit?)`	split by regex match till limit
String	`.replace(charSeq old, charSeq new)`	replaces all occurrences
int	`.indexOf(char/String seq, fromIndex?)`	returns index
String	`.valueOf(value)`	converts given type to string

Selected StringBuilder methods

Return	Method	Description
StringBuilder	`.append(String s)`	append string
StringBuilder	`.insert(int offset, String s)`	insert string at specified offset
StringBuilder	`.replace(start, end, str)`	replace string from start to end with str
StringBuilder	`.delete(start, end)`	delete from start to end
StringBuilder	`.reverse()`	reverse
String	`.substring(begin, end?)`	return substr

Note: StringBuffer is synchronized version of StringBuilder.

Collections

List, Queue, Set

List<Integer> list = new ArrayList<>();
Stack<Integer> stk = new Stack<>();
Queue<Integer> q = new ArrayDeque<>();
Set<Integer> s = new HashSet<>();

Return	Method	Description
boolean	`.add(E e)`	insert element
boolean	`.remove(Obj element)`	delete an element
void	`.clear()`	remove all elements
boolean	`.contains(Obj element)`	to search element
Iterator	`.iterator()`	returns iterator
void	`fill(List<?> list, T obj)`	fill all with obj
min, max, reverse, rotate, shuffle, sort.

Map<K,V>

Return	Method	Description
V	`.get(Object key)`	returns value mapped to key or null
V	`.getOrDefault(Object key, V defaultValue)`	if key not present return default
boolean	`.containsKey(Obj K)`	-
boolean	`.containsValue(Obj V)`	if key mapping to V exists
V	`.put(K, V)`	-
boolean	`.remove(K, V?)`	remove if key and optional value match
???	`.merge(K, V, remappingFunction)`	still confused

Iterate

Set, list, queue

Set<String> set = new HashSet<String>();
// 1 Enhanced For loop
for(String s: set) {
    System.out.println(s);
}
//2
set.forEach(System.out::println);
//3 Lambda
set.forEach((s) -> System.out.println(s));
//4 Iterator

Map

Algorithms

Sorting Sort Map or custom object based on more than one condition.
Eg: Sort map based on frequency of words in descending and ascending in lexicographical order if same frequency.

Map<String, Integer> count = new HashMap<>();
List<String> freqs = new ArrayList<>(count.keySet());

Collections.sort(
    freqs,
    (w1, w2) -> count.get(w1).equals(count.get(w2)) ?
    w1.compareTo(w2) :
    count.get(w2) - count.get(w1);
)

//Sort in reverse
Collections.sort(arr, Collections.reverseOrder());

Binary Search

Conversions

Collections - Array

To Array

Foo[] array = list.toArray(new Foo[0]);
//or
Foo[] array = list.stream().toArray(Foo[]::new);

To Collection

String[], Integer[] to List (from Class)

List<T> list = new ArrayList<T>(Arrays.asList(arrValues));

int[] to List (from Primitive)

List<Integer> list = new ArrayList<Integer>(intarr.length);
for(int i: intarr) {
    list.add(i);
}
//or
List<Integer> list = Arrays.stream(intarr).boxed().collect(Collections.toList());

Specifics

Copy array

arraycopy

When the array memory is already allocated. or want to reuse existing array.

public static void arraycopy(  
    src, int srcPos,dest, int destPos, int length  
)

Clone

If we create the clone of a single-dimensional array, it creates the deep copy of the Java array. It means, it will copy the actual value. But, if we create the clone of a multidimensional array, it creates the shallow copy of the Java array which means it copies the references.

int carr[]=arr.clone();

Convert to string

valueOf

If object is null returns "null"

String.valueOf(value);

toString() method

If object is null, null pointer exception

myint.toString()

priyesh18/java-lookup