工厂模式（计算器）

public abstract class Operation {
    private Integer numberA = 0;
    private Integer numberB = 0;

    public abstract double getResult();

    public Integer getNumberA() {
        return numberA;
    }
    public void setNumberA(Integer numberA) {
        this.numberA = numberA;
    }
    public Integer getNumberB() {
        return numberB;
    }
    public void setNumberB(Integer numberB) {
        this.numberB = numberB;
    }
}

public class OperationAdd extends Operation {
    @Override
    public double getResult(){
        return getNumberA() + getNumberB();
    }
}

public class OperationSub extends Operation {
    @Override
    public double getResult(){
        return getNumberA() - getNumberB();
    }
}

public class OperationFactory {
    public static Operation createOperation(String operate){
        Operation oper = null;
        if("+".equals(operate)){
            oper = new OperationAdd();
        }
        if("-".equals(operate)){
            oper = new OperationSub();
        }
        return oper;
    }
}

public class Client {
    public static void main(String[] args){
        Operation oper;
        oper = OperationFactory.createOperation("+");
        oper.setNumberA(1);
        oper.setNumberB(2);
        double result = oper.getResult();
        System.out.println(result);
    }
}