Common Compile Time and Runtime Error Messages
And What They Mean

===== incompatible type =====
The variable on the left side of the assignment operator (=) and the value on the right side have different types. For example, 

String str = 5;

The variable on the left is of type String and the variable on the right is of type int. You cannont convert one to the other.



===== cannot find symbol -- method nextString =====
This method doesn't exist in the class. Either you mis-spelled the method or you are incorrectly remembering the name of the method. Go to the class API and look at the available methods



===== cannot find symbol -- method lastIndexOf =====
This method does exist and you have spelled it correctly. There is another syntax error somewhere on this line. For example,

int position = lastIndexOf(' ');   // incorrect -- you need to use the dot operator
int position = name.lastIndexOf(' '); // correct



===== NullPointerException =====
An object was not initialized (and thus is null) and you used the dot operator to call a method
An array was not initialized (and thus is null) and you used the bracket operator to index []
An object in an array is not initialized (i.e. array[i] is null) and you used the dot operator

For example,
Random rng; // declared but not initialized. rng holds the value null
rng.nextInt(100); // you are calling the nextInt() method on a null variable