APCS Unit 3 Design a Class Project

Expression.java

/**
 * A class that models mathematical expressions.
 * 
 * @author Noble H. Mushtak
 * @version 1.0 (6 Oct 2016)
*/
public class Expression
{
    /**
     * These are the five operations allowed in our expressions.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public static enum Operation { PLUS, MULTIPLY, POWER };
    /**
     * These are the three types of expression.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public static enum Type { NUMBER, VARIABLE, OPERATION };
    
    /**
     * This is the type of this expression.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Type type;
    
    /**
     * This is null if type != NUMBER.
     * This is the integer this Expression represents otherwise.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Double value;
    
    /**
     * This is null if type != VARIABLE.
     * This is the name of the variable this Expression represents otherwise.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public String name;
    
    /**
     * This is null if type != OPERATION.
     * This is the Operation used in this expression otherwise.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Operation op;
    /**
     * This is null if type != OPERATION.
     * This is the list of operands that are being operated on
     * by the above op.
     * 
     * If op is PLUS or MULTIPLY, then operands.length >= 2.
     * Otherwise, operands.length == 2.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Expression[] operands;
    
    /**
     * This constructor takes an double and
     * returns an Expression representing a number.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Expression(Double realValue)
    {
        type = Expression.Type.NUMBER;
        value = realValue;
    }
    
    /**
     * This constructor takes a String and
     * return an Expression representing a variable.
     * 
     * Also, this takes out all of the non-letters out of the variable name.
     * 
     * @since 1.0 (6 Oct 2016)
    */
    public Expression(String variableName) {
        //This is the variable name after we take out all of the non-letters.
        String realVariableName = "";
        type = Expression.Type.VARIABLE;
        name = variableName;
    }
    
    /**
     * This tells us if this is a valid expression
     * where the operations satisfy the properties told above.
     * 
     * @returns boolean -- Is this expression valid?
     * @since 1.0 (6 Oct 2016)
    */
    public boolean validExpression()
    {
        //This is what we're going to return.
        boolean isValid = true;
        //Based on the type:
        switch (type) {
            //If number, everything null except value.
            case NUMBER:
                isValid &= (name == null);
                isValid &= (op == null);
                isValid &= (operands == null);
                break;
            //If variable, everything null except variableName.
            case VARIABLE:
                isValid &= (value == null);
                isValid &= (op == null);
                isValid &= (operands == null);
                break;
            //If operation, everything null except op and operands.
            case OPERATION:
                isValid &= (value == null);
                isValid &= (name == null);
                break;
        }
        
        return isValid;
    }
}