Array

Arrays.fill(array, value);

System.arraycopy(
    sourceArray,
    sourceStart,
    destinationArray,
    destinationStart,
    length
);

Map

map.getOrDefault(key, defaultVal);

map.putIfAbsent(key, value);

//smallest
treeMap.firstEntry();
// greatest key less than or equal to k
treeMap.floorKey(k);
// greatest entry with key less than or equal to k
treeMap.floorEntry(k);
// greatest key strictly less than k
treeMap.lowerKey(k);

Set

// smallest
treeSet.first();
// greatest element less than or equal to k
treeSet.floor(k);
// smallest element greater than or equal to k
treeSet.ceiling(k);
// strictly less / strictly greater
treeSet.lower(k);
treeSet.higher(k);

Queue

PriorityQueue<Integer> minHeap = new PriorityQueue<>();
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());

Bitwise Tricks

For negative odd values, >> n rounds differently from integer division by 2^n.

Swap Values:

int temporary = x;
x = y;
y = temporary;

The temporary-variable form is clearer and also works when both expressions refer to the same storage location.

Reference: Java Language Specification: bitwise and shift operators.