• equals
• hashCode
• Additional considerations

equals

• Use == operator to compare object references. This is the most performance comparison.
• Use instanceof to check that both objects have same type.
• Cast the argument to the class type.
• Check desired fields in the class to return true when you consider that two objects are equals.
• To compare primitive fields whose type is not double or float, use == operator in the comparison.
• To compare objects, use equals method.
• To compare float and double use Float.compare(float, float) and Double.compare(double, double) respectively.
• To improve performance, first compare fields that are more likely to differ or less expensive to compare.
• Add test unless you autogenerate or use annotations.

@Override 
public boolean equals(Object o) {

    if (o == this) return true;

    if (!(o instanceof MyObject)) return false;

    MyObject mo = (MyObject)o;

    return mo.mostImportantField == mostImportantField 
        && mo.sencondField == sencondField
        && mo.lessImportantField == lessImportantField;
}

hashCode

• Declare an integer variable named result and initialize it with the first field.
• For primitive fields use e.g. Integer.hashCode(field)
• If the field is an object, call hashCode recursively.
• If the field is an array use Arrays.hashCode.
• Combine all hash codes into the result and return result.
• Multiply result every time by 31 (odd prime which can be replaced by a shift and a subtraction for better performance).

@Override
public int hashCode() {
    int result = Integer.hashCode(firstField);
    result += 31 * result + sencondField.hashCode(); 
    result += 31 * result + Integer.hashCode(thirdField); 
    return result;
}

Additional considerations

• Always override HashCode when overriding equals.
• Always use Object type as an argument in the equals method.
• Use @Override annotation.
• If there is no need to create a custom equals or hashCode method you only need to use @Autovalue or @EqualsAndHashCode from Google and Lombok libraries respectively.
• Alternatively, you can use autogeneration provided by your IDE.

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