A method should perform one coherent operation and expose a clear contract. Java passes arguments by value. For object arguments, the copied value is the reference itself.

static int add(int a, int b) {
    return a + b;
}

static int sum(int... values) {
    int total = 0;
    for (int value : values) total += value;
    return total;
}

Varargs are compiled as an array parameter and must be the final parameter.

Arrays are fixed-size objects with zero-based indexing.

int[] values = new int[3];
values[0] = 10;

int[] scores = {90, 80, 95};

int[][] matrix = {
    {1, 2},
    {3, 4}
};

Java's multidimensional arrays are arrays of arrays, so rows can have different lengths. Arrays are efficient for indexed access, but collections are usually better when size or data-structure semantics vary.

String is immutable. Operations such as concatenation produce a new string rather than changing the existing instance.

String name = "Java";
String upper = name.toUpperCase();

StringBuilder builder = new StringBuilder();
for (int i = 0; i < 3; i++) {
    builder.append(i).append(',');
}

String result = builder.toString();

Repeated concatenation in a large loop can create unnecessary temporary strings. StringBuilder is generally appropriate for incremental construction in one thread.

equals() defines logical equality when overridden. hashCode() must be consistent with equals: equal objects must have equal hash codes. toString() provides a readable representation for logs and debugging.

public record UserKey(String tenantId, String userId) {}

For mutable classes used as HashMap keys, changing fields involved in equality after insertion can make the entry effectively unreachable.

The equality relation should be reflexive, symmetric, transitive, consistent, and false for comparison with null.