我想我根据 Touch 建议我的解释器模式的建议找到了一个优雅的解决方案。我以前不知道解释器模式，但在阅读了 15 分钟后，我可以想出一个很酷的解决方案。我以前从未获得过公认的答案，所以我希望我的答案能获得它。
逻辑很简单。我们必须使用“逆波兰表示法”，例如：
ab +
abc + -
ab + ca - -
&这就完成了。http://en.wikipedia.org/wiki/Interpreter_pattern
interface Expression {
public int interpret(Map<String,Expression> variables);
}
class Number implements Expression {
private int number;
public Number(int number) { this.number = number; }
public int interpret(Map<String,Expression> variables) { return number; }
}
class Plus implements Expression {
Expression leftOperand;
Expression rightOperand;
public Plus(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) + rightOperand.interpret(variables);
}
}
class Minus implements Expression {
Expression leftOperand;
Expression rightOperand;
public Minus(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) - rightOperand.interpret(variables);
}
}
class Times implements Expression {
Expression leftOperand;
Expression rightOperand;
public Times(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) * rightOperand.interpret(variables);
}
}
class Division implements Expression {
Expression leftOperand;
Expression rightOperand;
public Division(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return leftOperand.interpret(variables) / rightOperand.interpret(variables);
}
}
class IfThen implements Expression {
Expression leftOperand;
Expression rightOperand;
public IfThen(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables)==1) ? rightOperand.interpret(variables) : 0;
}
}
class GreaterThan implements Expression {
Expression leftOperand;
Expression rightOperand;
public GreaterThan(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables) > rightOperand.interpret(variables)) ? 1 : 0;
}
}
class GreaterThanOrEqual implements Expression {
Expression leftOperand;
Expression rightOperand;
public GreaterThanOrEqual(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables) >= rightOperand.interpret(variables)) ? 1 : 0;
}
}
class LessThan implements Expression {
Expression leftOperand;
Expression rightOperand;
public LessThan(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables) < rightOperand.interpret(variables)) ? 1 : 0;
}
}
class LessThanOrEqual implements Expression {
Expression leftOperand;
Expression rightOperand;
public LessThanOrEqual(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables) <= rightOperand.interpret(variables)) ? 1 : 0;
}
}
class Equal implements Expression {
Expression leftOperand;
Expression rightOperand;
public Equal(Expression left, Expression right) {
leftOperand = left;
rightOperand = right;
}
public int interpret(Map<String,Expression> variables) {
return (leftOperand.interpret(variables) == rightOperand.interpret(variables)) ? 1 : 0;
}
}
class Variable implements Expression {
private String name;
public Variable(String name) { this.name = name; }
public int interpret(Map<String,Expression> variables) {
if(null==variables.get(name)) return 0; //Either return new Number(0).
return variables.get(name).interpret(variables);
}
}
class Evaluator implements Expression {
private Expression syntaxTree;
public Evaluator(String expression) {
Stack<Expression> expressionStack = new Stack<Expression>();
for (String token : expression.split(" ")) {
if (token.equals("+")) {
Expression subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
expressionStack.push( subExpression );
}
else if (token.equals("-")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new Minus(left, right);
expressionStack.push( subExpression );
}
else if (token.equals("*")) {
Expression subExpression = new Times(expressionStack.pop(), expressionStack.pop());
expressionStack.push( subExpression );
}
else if (token.equals("/")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new Division(left, right);
expressionStack.push( subExpression );
}
else if (token.equals("if-then")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new IfThen(left, right);
expressionStack.push( subExpression );
}
else if (token.equals(">")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new GreaterThan(left, right);
expressionStack.push( subExpression );
}
else if (token.equals(">=")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new GreaterThanOrEqual(left, right);
expressionStack.push( subExpression );
}
else if (token.equals("<")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new LessThan(left, right);
expressionStack.push( subExpression );
}
else if (token.equals("<=")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new LessThanOrEqual(left, right);
expressionStack.push( subExpression );
}
else if (token.equals("==")) {
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.pop();
// ..and after the left one
Expression left = expressionStack.pop();
Expression subExpression = new Equal(left, right);
expressionStack.push( subExpression );
}
else
expressionStack.push( new Variable(token) );
}
syntaxTree = expressionStack.pop();
}
public int interpret(Map<String,Expression> context) {
return syntaxTree.interpret(context);
}
}
＆ 而已。要调用转换方法，我们只需要这个。看这个简单的代码：
String expression = "w x == z if-then";
//String expression = "w x - z +";
Evaluator sentence = new Evaluator(expression);
Map<String,Expression> variables = new HashMap<String,Expression>();
variables.put("w", new Number(10));
variables.put("x", new Number(5));
variables.put("z", new Number(42));
int result = sentence.interpret(variables);
System.out.println(result);
& 就完成了。但它有一个限制，它没有循环，但如果 then 对我来说已经足够了。
我做得对吗？