What criteria and conditions must fields in a class satisfy?

In Java, class fields should meet the following criteria and conditions:

• Initialization: fields can be initialized at declaration or in the class constructor.
• Access modifiers: usually fields are made private to encapsulate data, with access managed through getter and setter methods.
• Data types: fields must have a defined type (primitive or reference).
• Constants: if a field should be immutable, it is declared with final and static modifiers (e.g., constants).
• Static fields: if a field belongs to the class as a whole, not to a specific object, it is declared as static.

Example:

public class Person {
    private String name; // private field
    private int age;
    public static final String SPECIES = "Homo sapiens"; // constant

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // getters and setters
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
}