Question: Starting with the BinaryExpression learning activity solution from this week, Code a new class DebugVisitor that visits an expression tree and returns a list of the calls made to the visitor. For example, if the expression tree represented the expression 3 + 5, then the output would be:

preVisit AddExpression

visit ConstantExpression 3

visit AddExpression

visit ConstantExpression 5

postVisit AddExpression

Hint: You can use obj.getClass().getSimpleName() to get the name of an object.

AddExpression.java

public class AddExpression extends BinaryExpression

{

public AddExpression(Expression left, Expression right)

{

super(left, right);

}

@Override

public Operation getOperation()

{

return Operation.ADD;

}

}

BinaryExpression.java

public abstract class BinaryExpression extends Expression

{

Expression leftChild;

Expression rightChild;

public BinaryExpression(Expression left, Expression right) {

leftChild = left;

rightChild = right;

}

@Override

public void accept(Visitor visitor)

{

visitor.preVisit(this);

leftChild.accept(visitor);

visitor.visit(this);

rightChild.accept(visitor);

visitor.postVisit(this);

}

public abstract Operation getOperation();

}

ConstantExpression.java

public class ConstantExpression extends Expression

{

Integer value;

public ConstantExpression(Integer value) {

this.value = value;

}

public Integer getValue() {

return value;

}

@Override

public void accept(Visitor visitor)

{

visitor.visit(this);

}

}

DebugVisitor.java

import java.util.*;

public class DebugVisitor implements Visitor {

public DebugVisitor() {

}

public List getResult() {

return null;

}

@Override

public void preVisit(BinaryExpression expr) {

}

@Override

public void visit(BinaryExpression expr) {

}

@Override

public void postVisit(BinaryExpression expr) {

}

@Override

public void visit(ConstantExpression expr) {

}

}

DivideExpression.java

public class DivideExpression extends BinaryExpression

{

public DivideExpression(Expression left, Expression right)

{

super(left, right);

}

@Override

public Operation getOperation()

{

return Operation.DIVIDE;

}

}

Expression.java

public abstract class Expression implements Visitable

{

}

MultiplyExpression.java

public class MultiplyExpression extends BinaryExpression

{

public MultiplyExpression(Expression left, Expression right)

{

super(left, right);

}

@Override

public Operation getOperation()

{

return Operation.MULTIPLY;

}

}

Operation.java

public abstract class Operation

{

private String value;

private Operation(String str)

{

this.value = str;

}

public abstract Integer apply(Integer left, Integer right);

public String toString()

{

return this.value;

}

public static final Operation ADD = new Operation("+") {

@Override

public Integer apply(Integer left, Integer right)

{

return left.intValue() + right.intValue();

}

};

public static final Operation SUBTRACT = new Operation("-") {

@Override

public Integer apply(Integer left, Integer right)

{

return left.intValue() - right.intValue();

}

};

public static final Operation MULTIPLY = new Operation("*") {

@Override

public Integer apply(Integer left, Integer right)

{

return left.intValue() * right.intValue();

}

};

public static final Operation DIVIDE = new Operation("/") {

@Override

public Integer apply(Integer left, Integer right)

{

return left.intValue() / right.intValue();

}

};

}

SubtractExpression.java

public class SubtractExpression extends BinaryExpression

{

public SubtractExpression(Expression left, Expression right)

{

super(left, right);

}

@Override

public Operation getOperation()

{

return Operation.SUBTRACT;

}

}

Visitable.java

public interface Visitable

{

void accept(Visitor visitor);

}

Visitor.java

public interface Visitor

{

public void preVisit(BinaryExpression expr);

public void visit(BinaryExpression expr);

public void postVisit(BinaryExpression expr);

public void visit(ConstantExpression expr);

}