如何将这个简单的计算器程序中的开关/案例语句制作成跳转表。
import java.lang.*;
import java.util.*;
public class Calculator
{
private int solution;
private static int x, y, ops;
private char operators;
public Calculator()
{
solution = 0;
}
public int addition(int x, int y)
{
return x + y;
}
public int subtraction(int x, int y)
{
return x - y;
}
public int multiplication(int x, int y)
{
return x * y;
}
public int division(int x, int y)
{
solution = x / y;
return solution;
}
public void calc(int ops){
Scanner operands = new Scanner(System.in);
System.out.println("operand 1: ");
x = operands.nextInt();
System.out.println("operand 2: ");
y = operands.nextInt();
System.out.println("Solution: ");
switch(ops)
{
case(1):
System.out.println(addition(x, y));
break;
case(2):
System.out.println(subtraction(x, y));
break;
case(3):
System.out.println(multiplication(x, y));
break;
case(4):
System.out.println(division(x, y));
break;
}
}
public static void main (String[] args)
{
System.out.println("What operation? ('+', '-', '*', '/')");
System.out.println(" Enter 1 for Addition");
System.out.println(" Enter 2 for Subtraction");
System.out.println(" Enter 3 for Multiplication");
System.out.println(" Enter 4 for Division");
Scanner operation = new Scanner(System.in);
ops = operation.nextInt();
Calculator calc = new Calculator();
calc.calc(ops);
}
}
老实说,我不知道跳转表到底是什么(在网上找不到任何解释(,所以我不知道它与开关/案例语句有何不同。
旁注:此代码仅处理整数,因此如果您除以 5/3,它会给您 1。我怎样才能轻松地将其更改为浮点数/双倍数。
如前所述,跳转表是指向函数的偏移量/指针数组。 与C/C++不同,Java并没有真正的函数指针(Java中的函数指针(。
但是你可以假装,并以面向对象的方式进行。 使用一个方法 (f( 定义基类 (Funky(。 派生多个子项,每个函数操作一个(+,-,*,/等(,并为每个子项创建一个对象(毕竟它只是一个接口(,并将该子项存储到类型数组(Funky(中。
在表中查找操作,并对参数调用该方法
例:
定义一个基类(或一个接口,这会让你更快乐?(。 请注意,如果扩展类,则可以使用基类方法作为默认值(生成错误消息或引发异常(,
public class X
//or, public interface X
{
//method
Z fun(Z z1, Z z2)
{
//nothing to see here
}
}
class X1 extends X //or, implements X
{
public Z fun(Z z1, Z z2)
{
//variant1 stuff here
}
}
...
public class Xn extends X //or, implements X
{
public Z fun(Z z1, Z z2)
{
//variantn stuff here
}
}
哦,您将需要实例,并将它们加载到数组(跳转表(中。
有些技术对某些语言来说是惯用的,跳跃表更像是一个系统的东西,而不是一个Java的东西,而不是一个真正的Java习语。
好吧,我不知道什么是跳转表,但是如果您想控制其他类型的数字,则可以更改参数,例如您的方法:
public int addition(int x, int y)
{
return x + y;
}
如果你想双>
public int addition(Double x, Double y)
但我强烈建议您使用类型"数字",每隔一个类就会从"数字"扩展。
编号.class
前任:
public static String numeroToLetra(Number num)
{
Integer numero = Integer.valueOf(num.intValue()); //int value
Double numero = Double.valueOf(num.doubleValue());........
}//so you can pass whatever type of number.
这是一个古老的问题,但我认为它仍然有价值来说明自Java 8以来你可以做什么。基本上,您创建一个接口,其唯一目的是为操作数组提供类型,然后使用方法引用来填充操作数组。之后,您可以使用索引来选择正确的操作。我对 OP 的代码进行了最小的修改,以便比较最简单:
import java.util.Scanner;
public class Calculator
{
//
// Create an interface to use as Type for
// operations array.
//
private interface BinaryOperation {
int performOperation(int a, int b);
}
//
// Array has one unused element to make coding easier
// and use operation as a direct index.
// You can replace with 4 element array easily.
//
BinaryOperation[] operations = new BinaryOperation[5];
private int solution;
private static int x, y, ops;
private char operators;
public Calculator()
{
solution = 0;
//
// Initialize jump table using method references.
//
operations[1] = this::addition;
operations[2] = this::subtraction;
operations[3] = this::multiplication;
operations[4] = this::division;
}
public int addition(int x, int y)
{
return x + y;
}
public int subtraction(int x, int y)
{
return x - y;
}
public int multiplication(int x, int y)
{
return x * y;
}
public int division(int x, int y)
{
solution = x / y;
return solution;
}
public void calc(int ops){
Scanner operands = new Scanner(System.in);
System.out.println("operand 1: ");
x = operands.nextInt();
System.out.println("operand 2: ");
y = operands.nextInt();
System.out.println("Solution: ");
//
// Call binary operation through jump table
//
System.out.println(operations[ops].performOperation(x, y));
}
public static void main (String[] args)
{
System.out.println("What operation? ('+', '-', '*', '/')");
System.out.println(" Enter 1 for Addition");
System.out.println(" Enter 2 for Subtraction");
System.out.println(" Enter 3 for Multiplication");
System.out.println(" Enter 4 for Division");
Scanner operation = new Scanner(System.in);
ops = operation.nextInt();
Calculator calc = new Calculator();
calc.calc(ops);
}
}
如果您使用的是支持 lambda 的 Java 版本,则更符合实现为"跳转表"要求的解决方案将使用实际的跳转表,该跳转表将运算符代码映射到实现每个操作数的 lambda 表达式。
这是一种令人愉快的方法,不仅可以消除笨拙的 switch 语句,还可以生成更易于维护且更易于扩展的代码。以后可以轻松添加新操作数,而无需对计算器实现进行任何更改。只需实现 new 运算符及其命名方法,并将其添加到跳转表中即可。您的计算器将自动支持新的操作数。
import com.google.common.collect.ImmutableMap;
import java.lang.*;
import java.util.*;
public class Calculator
{
private static final Map<Integer,BinaryOperator<Integer>> evaluators = ImmutableMap.<Integer, BinaryOperator<Integer>>builder()
.put(1, (Integer x, Integer y) -> new IntAddition().evaluateFor(x,y))
.put(2, (Integer x, Integer y) -> new IntSubtraction().evaluateFor(x,y))
.put(3, (Integer x, Integer y) -> new IntMultiplication().evaluateFor(x,y))
.put(4, (Integer x, Integer y) -> new IntDivision().evaluateFor(x,y))
.build();
private static final Map<Integer,Nameable> names = ImmutableMap.<Integer, Nameable>builder()
.put(1, () -> new IntAddition().getName())
.put(2, () -> new IntSubtraction().getName())
.put(3, () -> new IntMultiplication().getName())
.put(4, () -> new IntDivision().getName())
.build();
private int solution;
private static int x, y, ops;
public Calculator()
{
solution = 0;
}
public void calc(int opcode)
{
Scanner operands = new Scanner(System.in);
System.out.println("Enter operand 1: ");
x = operands.nextInt();
System.out.println("Enter operand 2: ");
y = operands.nextInt();
System.out.print("Solution: ");
System.out.println(evaluators.get(opcode).evaluateFor(x, y));
}
public static void main(String[] args)
{
System.out.println("What operation?");
for (Integer opcode : evaluators.keySet())
{
System.out.println(String.format(" Enter %d for %s", opcode, names.get(opcode).getName()));
}
Scanner operation = new Scanner(System.in);
ops = operation.nextInt();
Calculator calc = new Calculator();
calc.calc(ops);
}
interface Nameable
{
String getName();
}
interface BinaryOperator<T>
{
T evaluateFor(T x, T y);
}
static class IntAddition implements BinaryOperator<Integer>, Nameable
{
IntAddition() { }
public Integer evaluateFor(Integer x, Integer y)
{
return x + y;
}
public String getName()
{
return "Addition";
}
}
static class IntSubtraction implements BinaryOperator<Integer>, Nameable
{
IntSubtraction() { }
public Integer evaluateFor(Integer x, Integer y)
{
return x - y;
}
public String getName()
{
return "Subtraction";
}
}
static class IntMultiplication implements BinaryOperator<Integer>, Nameable
{
IntMultiplication() { }
public Integer evaluateFor(Integer x, Integer y)
{
return x * y;
}
public String getName()
{
return "Multiplication";
}
}
static class IntDivision implements BinaryOperator<Integer>, Nameable
{
IntDivision() { }
public Integer evaluateFor(Integer x, Integer y)
{
return x / y;
}
public String getName()
{
return "Division";
}
}
}