JAVA计算器的良好实现

//接口
package com.czp;

import java.math.BigDecimal;

public interface IOperator {

    BigDecimal operator(BigDecimal n1, BigDecimal n2);
    
    BigDecimal operator(int n1, int n2);
    
    BigDecimal operator(double n1, double n2);
}
//工厂类

package com.czp;

import java.math.BigDecimal;


/**
 * 获取操作的工厂类
 * 
 * @author Czp
 * @version 1.0 (2013-6-28)
 */
public class OperatorFatory {

    private static final OperatorFatory FATORY = new OperatorFatory();

    private IOperator add;

    private IOperator sub;

    private IOperator mul;

    private IOperator div;

    private OperatorFatory() {
        add = new AddOperator();
        sub = new SubOperator();
        mul = new MulOperator();
        div = new DivOperator();
    }

    /**
     * 操作的类型
     * 
     * @author Czp
     * @version 1.0 (2013-6-28)
     */
    public enum TYPE {
        ADD("+"), SUB("-"), MUL("*"), DIV("/");
        
        String type;

        private TYPE(String type) {
            this.type = type;
        }
        
        static TYPE fromString(String str)
        {
            TYPE[] tps = TYPE.values();
            for (TYPE type : tps) {
                if(type.type.equals(str.trim()))
                {
                    return type;
                }
            }
            return null;
        }
        
    }

    /**
     * 获取工厂的实例
     * 
     * @return
     */
    public static OperatorFatory getInstance() {
        return FATORY;
    }

    /**
     * operator的工厂方法
     * 
     * @param type
     * @return
     */
    public IOperator getOperator(TYPE type) {
        switch (type) {
        case ADD:
            return add;
        case SUB:
            return sub;
        case MUL:
            return mul;
        case DIV:
            return div;
        }
        return null;
    }
    /**
     * 通过字符串获取操作类
     * 
     * @param type
     * @return
     */
    public IOperator getOperator(String type) {
        return getOperator(TYPE.fromString(type));
    }
    
    /************** 以下是+-x/的具体的实现 ***********************/

    private static abstract class AbstractOperator implements IOperator {

        @Override
        public abstract BigDecimal operator(BigDecimal n1, BigDecimal n2);

        @Override
        public BigDecimal operator(int n1, int n2) {
            return operator(BigDecimal.valueOf(n1), BigDecimal.valueOf(n2));
        }

        @Override
        public BigDecimal operator(double n1, double n2) {
            return operator(BigDecimal.valueOf(n1), BigDecimal.valueOf(n2));
        }

    }

    private static class AddOperator extends AbstractOperator {

        @Override
        public BigDecimal operator(BigDecimal n1, BigDecimal n2) {
            return n1.add(n2);
        }

    }

    private static class DivOperator extends AbstractOperator {

        private static final BigDecimal ZERO = new BigDecimal(0);

        @Override
        public BigDecimal operator(BigDecimal n1, BigDecimal n2) {
            if (n2.compareTo(ZERO) == 0)
                throw new RuntimeException("second number is zero");
            return n1.divide(n2);
        }

    }

    private static class SubOperator extends AbstractOperator {

        @Override
        public BigDecimal operator(BigDecimal n1, BigDecimal n2) {
            return n1.subtract(n2);
        }

    }

    private static class MulOperator extends AbstractOperator {

        @Override
        public BigDecimal operator(BigDecimal n1, BigDecimal n2) {
            return n1.multiply(n2);
        }

    }
}