JAVA OOP concept issue – Code Review Stack Exchange

Hello guys could you help me please how to render this code as OOP?

I want to improve this code and follow the OOP anyone could help me, please

thank you in advance

this is class Expression

class Expression {
        public int type;
        public int value;
        public Expression leftOp;
        public Expression rightOp;
        public Expression(int type, int value, Expression leftOp, Expression rightOp) {
            this.type = type;
            this.value = value;
            this.leftOp = leftOp;
            this.rightOp = rightOp;

    class Arith {

/** Constantes pour representer les types */
        public static final int TYPE_NUMBER = 1;
        public static final int TYPE_SUM = 2;
        public static final int TYPE_PROD = 3;


    public static void main(String() args) {
            Expression term = new Expression(TYPE_SUM, 0, new Expression(TYPE_NUMBER, 3, null, null), new Expression(
                    TYPE_PROD, 0, new Expression(TYPE_NUMBER, 2, null, null), new Expression(TYPE_NUMBER, 5, null, null)));
        /** Evalue recursivement the expression */
        public static int evaluate(Expression term) {
            switch (term.type) {
            case TYPE_NUMBER:
                return term.value;
            case TYPE_SUM:
                return evaluate(term.leftOp) + evaluate(term.rightOp);
            case TYPE_PROD:
                return evaluate(term.leftOp) * evaluate(term.rightOp);
                return 0;