Common Mistakes

Forgetting to declare your variables.

Histo.java:21: cannot find symbol
symbol  : variable count
location: class Histo
        count = new ArrayList<Integer>(10);
        ^

Not importing a class:

Histo.java:9: cannot find symbol
symbol  : class Scanner
location: class Histo
        Scanner data = null;
        ^

Forgetting to use the new keyword to create an object.

Here’s an example of the error message that occurs when you forget to use the new keyword. Notice that the message is pretty unhelpful. Java thinks you are trying to call the Method Scanner, but there are two problems. First Scanner is not really a method it is a constructor.:

Histo.java:14: cannot find symbol
symbol : method Scanner(java.io.File)
location: class Histo
data = Scanner(new File("test.dat"));

Forgetting a Semicolon:

Histo.java:19:
';' expected
System.exit(0);
^

Forgetting to declare the kind of object in a container.:

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