Java With Us

The <code>new</code> Keyword in Java

Updated on 2026-04-21 Reviewed by the Java With Us editorial team

new creates a new object or array. It allocates memory, calls the constructor, and returns a reference. For every non-primitive value in Java, new is either in your code or hidden behind a factory method like List.of(...).

Creating objects

User u = new User("Alice", 30);
var list = new ArrayList<String>();        // var + diamond = tight syntax
var map  = new HashMap<String, Integer>();

The diamond operator (Java 7+)

List<String> old = new ArrayList<String>();   // redundant type argument
List<String> mod = new ArrayList<>();         // inferred from the left-hand side

Arrays

int[] a = new int[10];                      // all zeros
int[] b = new int[]{1, 2, 3};                // with initialiser
int[] c = {1, 2, 3};                         // short form — only at declaration
int[][] matrix = new int[4][4];              // 2-D array

Anonymous classes

Runnable r = new Runnable() {
    @Override public void run() { System.out.println("hi"); }
};
// Modern: lambda when the type is a functional interface
Runnable r = () -> System.out.println("hi");

Factories often replace new

List.of("a", "b", "c");                     // no 'new'
Map.of("a", 1, "b", 2);
Path.of("src", "main");
Instant.now();
Optional.of(value);

Prefer factories: they can return an immutable instance, an interned object, or the most appropriate implementation type.

new isn't the only way

  • Primitives aren't created with new — they're values.
  • String literals ("hello") come from the string pool, not new String("hello").
  • Enum constants are created once at class load.
  • Singletons intentionally avoid new after the first call.

Common mistakes

  • new String("literal") — creates an unnecessary second object. Just use "literal".
  • new Boolean(true) — deprecated since Java 9. Use Boolean.TRUE.
  • Raw types — new ArrayList() without type argument is legal but defeats generics. Add <>.
  • Allocating in a hot loop — garbage collector pressure. Reuse or use a pool where it matters (rare in modern JVMs).

Related

Pillar: Java keywords. See also constructors, classes.

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