Java Programming

im writing an expression tree for my comp sci class and everything works except the part i coppied directly out of the book.

public String toString()
{
return toString(root, 0);
}
private String toString(Node<String> tree, int level)
{
String str="";
if(tree!=null)
{
str+= toString(tree.right, level+1);
for(int i=1; i<=level; i++)
str=str+"|";
str+=tree.value.toString()+"\n";
str+=toString(tree.left, level+1);
}
return str;
}
this is the to string method that should print the tree like this:
| | x
| x |
| | x
x | |
| | x
| x |
| | x
but the computer doesnt like it when i compile it.

LinkedBSTPT.java:90: toString() in LinkedBSTPT cannot override toString() in
java.lang.Object; attempting to use incompatible return type
found : String
required: java.lang.String
public String toString()
^
LinkedBSTPT.java:96: incompatible types
found : java.lang.String
required: String
String str="";
^
LinkedBSTPT.java:99: operator + cannot be applied to String,String
str+= toString(tree.right, level+1);
^
LinkedBSTPT.java:101: incompatible types
found : java.lang.String
required: String
str=str+"|";
^
LinkedBSTPT.java:102: incompatible types
found : String
required: java.lang.String
str+=tree.value.toString()+"\n";
^
LinkedBSTPT.java:103: operator + cannot be applied to String,String
str+=toString(tree.left, level+1);
^
these are the errors it give me. some one please explain to me why the computer doesnt like this chunk of code!