Java interview question: In Java 8 count frequency of chars in a String. In this post we’ll show two implementations – using Map.merge() and Streams and Collectors.

In Java 8 count frequency of numbers/elements/objects. You may encounter this problem on Java job interviews. Here we implement it using Java 8 Collectors.

Java 8 reduce is a great method to compute a value from stream of values – reduce collection to value. In this post we’ll explain how reduce in Java work.

java.util.Optional.map(Function) The method Optional.map(Function) applies the given function to the optional value if it is present and returns an optional with the result or empty optional.

java.util.Optional.ofNullable(T) The method Optional.ofNullable(T) returns Optional containing given value if it was not null or an empty Optional in case of null value.

java.util.Map.replaceAll(BiFunction) The method Map.replaceAll(BiFunction) replaces each value with the result of applying the given function on the corresponding entry. Rethrows exceptions thrown by the replacement function.

java.util.Map.replace(key, oldValue, newValue) The method Map.replace(key, oldValue, newValue) replaces the value associated with the given key only if it is currently mapped to the oldValue.