Implemented expression tree as follows in java using stacks(LinkedList), but it has errors

I was trying to implement expression tree by using stacks(LinkedList) and I have the following error when I compile it.

Note: ExpressionTree.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

import java.util.LinkedList;
public class ExpressionTree implements ExpressionTreeInterface{
private ExpressionNode root;
public ExpressionTree(String expression){
LinkedList l = new LinkedList();
String[] s = expression.split(" ");
int i=0;
for (String a:s){
if (a == "0" || a == "1" || a == "2" || a == "3" ||a == "4" ||a == "5" ||
a == "6" ||a == "7" ||a == "8" ||a == "9"){
int b = Integer.parseInt(a);
ExpressionNode e = new ExpressionNode();
e.operand(b);
l.add(e);
}
else if (a == "*" || a == "/" || a == "+" || a == "-"){
ExpressionNode e = new ExpressionNode();
e.operator(a);
e.right = (ExpressionNode)l.get(l.size()-1);
e.left = (ExpressionNode)l.get(l.size()-2);
l.remove(l.size()-1);
l.remove(l.size()-2);
l.add(e);
}
}
root = (ExpressionNode)l.get(0);
}
public int eval(){
return eval(root);
}
public int eval(ExpressionNode r){
if (r.left!=null && r.right!=null){
if (r.getOperator() == "+"){
return eval(r.left) + eval(r.right);
} else if (r.operator == "*"){
return eval(r.left) * eval(r.right);
} else if (r.operator == "/"){
return eval(r.left) / eval(r.right);
} else {
return eval(r.left) - eval(r.right);
}
} else {
return r.getOperand();
}
}
private static class ExpressionNode{
int operand;
String operator;
ExpressionNode left;
ExpressionNode right;
public void operator(String operator){
this.operator = operator;
}
public void operand(int operand){
this.operand = operand;
}
public String getOperator(){
return operator;
}
public int getOperand(){
return operand;
}
}
public String postfix(){
return postfix(root).trim();
}
private String postfix(ExpressionNode s){
if (s.left == null && s.right == null){
return Integer.toString(s.getOperand());
} else{
return postfix(s.left) + " " + postfix(s.right) + " " + s.getOperator();
}
}
public String prefix(){
return prefix(root).trim();
}
private String prefix(ExpressionNode s){
if (s.left == null && s.right == null){
return Integer.toString(s.getOperand());
} else{
return s.getOperator() + " " + prefix(s.left) + " " + prefix(s.right);
}
}
public String infix(){
return infix(root).trim();
}
private String infix(ExpressionNode s){
if (s.left == null && s.right == null){
return Integer.toString(s.getOperand());
} else{
return infix(s.left) + " " + s.getOperator() + " " + infix(s.right);
}  
}
}

Also, I created the following test class but it again has the following error.

at java.util.LinkedList.checkElementIndex(LinkedList.java:555)
at java.util.LinkedList.get(LinkedList.java:476)
at ExpressionTree.<init>(ExpressionTree.java:27)
at Testfile.main(Testfile.java:4)

public class Testfile {
public static void main(String[] args){
String a = "34 2 - 5 *";
ExpressionTree s = new ExpressionTree(a);
System.out.println(s.eval());
}
}

Where did I do wrong? I think I used a lot of recursion, so maybe there was a mistake from there, but I cannot figure out where.

There are few issues in your code :

First issue is You can't compare string in java using == operators you need can use function like .equals() to compare two string values. So, this code fragement :

if (a == "0" || a == "1" || a == "2" || a == "3" ||a == "4" ||a == "5" ||
a == "6" ||a == "7" ||a == "8" ||a == "9")

gets changed to :

 if (a.equals("0")|| a.equals("1") ||a.equals("2") || a.equals("3") ||a.equals("4") ||a.equals("5") ||
a.equals("6") ||a.equals("7") ||a.equals("8") ||a.equals("9"))

same goes with this part of code fragment :

else if (a == "*" || a == "/" || a == "+" || a == "-")

you need to use .equals function in this as well.

Same issue is there in eval function as well in this part :

if (r.left!=null && r.right!=null){
if (r.getOperator() == "+"){
return eval(r.left) + eval(r.right);
} else if (r.operator == "*"){
return eval(r.left) * eval(r.right);
} else if (r.operator == "/"){
return eval(r.left) / eval(r.right);
} else {
return eval(r.left) - eval(r.right);
}
} else {
return r.getOperand();
}

Second issue is in input expression string ("34 2 - 5 *") one of the string is 34 but it wont satisfy this condition :

  if (a == "0" || a == "1" || a == "2" || a == "3" ||a == "4" ||a == "5" ||
a == "6" ||a == "7" ||a == "8" ||a == "9")

So, instead of checking whether your string is number or not by this way you can create helper function to do the same like this and call the same :

 private boolean checkStringIsNumber(String s) {
boolean numeric = true;
try {
Integer num = Integer.parseInt(s);
} catch (NumberFormatException e) {
numeric = false;
}
return numeric;
}

Third issue is in these two lines :

 l.remove(l.size()-1);
l.remove(l.size()-2);

Suppose your link list has two node at this point . After l1, l2. After executing first statement

l.remove(l.size()-1);

updated link list will contain one node i.e. l1. Now if you execute this statement

 l.remove(l.size()-2);

It will throw exception as link list size is now 1 and l.size()-2 will return in -1 which is invalid index;

So, I suppose you want to remove last 2 nodes so you can do instead of these two statements this :

 l.remove(l.size()-1);
l.remove(l.size()-1);

So, after resolving all these issues , updated code of ExpressionTree constructor is :

 public ExpressionTree(String expression){
LinkedList l = new LinkedList();
String[] s = expression.split(" ");
int i=0;
for (String a:s){
if (checkStringIsNumber(a)){
int b = Integer.parseInt(a);
ExpressionNode e = new ExpressionNode();
e.operand(b);
l.add(e);
}
else if (a.equals("*") || a.equals("/") || a.equals("+") || a.equals("-")){
ExpressionNode e = new ExpressionNode();
e.operator(a);
e.right = (ExpressionNode)l.get(l.size()-1);
e.left = (ExpressionNode)l.get(l.size()-2);
l.remove(l.size()-1);
l.remove(l.size()-1);
l.add(e);
}
}
root = (ExpressionNode)l.get(0);
}

and updated code of eval function is :

 public int eval(ExpressionNode r){
if (r.left!=null && r.right!=null){
if (r.getOperator().equals("+")){
return eval(r.left) + eval(r.right);
} else if (r.operator.equals("*")){
return eval(r.left) * eval(r.right);
} else if (r.operator.equals("/")){
return eval(r.left) / eval(r.right);
} else {
return eval(r.left) - eval(r.right);
}
} else {
return r.getOperand();
}
}

After making these changes above expression string works fine and gives result as 160.