Unit 25: Unchecked Warnings

Learning Objectives

After this unit, students should:

be aware of how to use generics with an array
be aware of unchecked warnings that compilers can give when we are using generics
be able to make arguments as to why a piece of code is type-safe for simple cases
know how to suppress warnings from compilers
be aware of the ethics when using the @SuppressWarnings("unchecked") annotation
know what is a raw type
be aware that raw types should never never be used in modern Java

Creating Arrays with Type Parameters

We have seen how arrays and generics do not mix well. One way to get around this is to use Java Collections, a library of data structures provided by Java, instead of arrays, to store our items. The ArrayList class provides similar functionality to an array, with some performance overhead.

ArrayList<Pair<String,Integer>> pairList;
pairList = new ArrayList<Pair<String,Integer>>(); // ok

pairList.add(0, new Pair<Double,Boolean>(3.14, true));  // error

ArrayList<Object> objList = pairList;  // error

ArrayList itself is a generic class, and when parameterized, it ensures type safety by checking for appropriate types during compile time. We can't add a Pair<Double,Boolean> object to a list of Pair<String,Integer>. Furthermore, unlike Java array, which is covariant, generics are invariant. There is no subtyping relationship between ArrayList<Object> and ArrayList<Pair<String,Integer>> so we can't alias one with another, preventing the possibility of heap pollution.

Using ArrayList instead of arrays only gets around the problem of mixing arrays and generics, as a user. ArrayList is implemented with an array internally after all. As computing students, especially computer science students, it is important to know how to implement your own data structures instead of using ones provided by Java or other libraries.

Let's try to build one and to minimise confusion let's call it Seq<T>:

Seq<T> v0.1 with getArray
class Seq<T> {
  private T[] array;

  public Seq(int size) {
    this.array = (T[]) new Object[size];
  }

  public void set(int index, T item) {
    this.array[index] = item;
  }

  public T get(int index) {
    return this.array[index];
  }

  public T[] getArray() {
    return this.array;
  }
}

This generic class is a wrapper around an array of type T. Recall that we can't instantiate a generic type directly (i.e. new T[]). On Line 6, to get around this restriction, we instantiate (new) an Object array instead, and then cast it to an array of T[] instead.

The code now compiles, but we receive the following message:

$ javac Seq.java
Note: Seq.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Let's do what the compiler tells us, and compile with the `-Xlint:unchecked" flags.

$ javac -Xlint:unchecked Seq.java
Seq.java:6: warning: [unchecked] unchecked cast
    array = (T[]) new Object[size];
                  ^
  required: T[]
  found:    Object[]
  where T is a type-variable:
    T extends Object declared in class Seq
1 warning

We get a warning that our Line 6 is doing an unchecked cast.

Unchecked Warnings

An unchecked warning is basically a message from the compiler that it has done what it can, and because of type erasures, there could be a run-time error that it cannot prevent. Recall that type erasure generates the following code:

(String) array.get(0);

Since array is an array of Object instances and the Java array is covariant, the compiler can't guarantee that the code it generated is safe anymore.

Consider the following:

Seq<String> seq = new Seq<String>(4);
Object[] objArray = seq.getArray();
objArray[0] = 4;
seq.get(0);  // ClassCastException

The last line would generate a ClassCastException, exactly a scenario that the compiler has warned us.

It is now up to us humans to change our code so that the code is safe. Suppose we remove the getArray method from the Seq class,

Seq<T> v0.2 without getArray
class Seq<T> {
  private T[] array;

  public Seq(int size) {
    this.array = (T[]) new Object[size];
  }

  public void set(int index, T item) {
    this.array[index] = item;
  }

  public T get(int index) {
    return this.array[index];
  }
}

Can we prove that our code is type-safe? In this case, yes. Since array is declared as private, the only way someone can put something into the array is through the Seq::set method¹. Seq::set only put items of type T into array. So the only type of objects we can get out of array must be of type T. So we, as humans, can see that casting Object[] to T[] is type-safe.

If we are sure (and only if we are sure) that the line

1	`array = (T[]) new Object[size];`

is safe, we can thank the compiler for its warning and assure the compiler that everything is going to be fine. We can do so with the @SuppressWarning("unchecked") annotation.

Seq<T> v0.3 with @SuppressWarnings
class Seq<T> {
  private T[] array;

  public Seq(int size) {
    // The only way we can put an object into array is through
    // the method set() and we only put object of type T inside.
    // So it is safe to cast `Object[]` to `T[]`.
    @SuppressWarnings("unchecked")
    T[] a = (T[]) new Object[size];
    this.array = a;
  }

  public void set(int index, T item) {
    this.array[index] = item;
  }

  public T get(int index) {
    return this.array[index];
  }
}

@SuppressWarning is a powerful annotation that suppresses warning messages from compilers. Like everything that is powerful, we have the responsibility to use it properly:

@SuppressWarning can apply to declaration at a different scope: a local variable, a method, a type, etc. We must always use @SuppressWarning to the most limited scope to avoid unintentionally suppressing warnings that are valid concerns from the compiler.
We must suppress a warning only if we are sure that it will not cause a type error later.
We must always add a note (as a comment) to fellow programmers explaining why a warning can be safely suppressed.

Note that since @SuppressWarnings cannot apply to an assignment but only to the declaration, we declare a local variable a in the example above before assigning this.array to a.

Raw Types

Another common scenario where we can get an unchecked warning is the use of raw types. A raw type is a generic type used without type arguments. Suppose we do:

Seq s = new Seq(4);

The code would compile perfectly. We are using the generic Seq<T> as a raw type Seq. Java allows this code to compile for backward compatibility. This is anyway what the code looks like after type erasure and how we would write the code in Java before version 5. Without a type argument, the compiler can't do any type-checking at all. We are back to the uncertainty that our code could bomb with ClassCastException after it ships.

Mixing raw types with parameterized types can also lead to errors. Consider:

Seq<String> s = new Seq<String>(4);
populateSeq(s);
String str = s.get(0);

where the method populateSeq uses raw types:

void populateSeq(Seq s) {
    s.set(0, 1234);
}

Since we use raw types in this method, the compiler can't help us. It will warn us:

Seq.java:24: warning: [unchecked] unchecked call to set(int,T) as a member of the raw type Seq
    s.set(0, 1234);
         ^
  where T is a type-variable:
    T extends Object declared in class Seq
1 warning

If we ignore this warning or worse, suppress this warning, we will get a run-time error when we execute a.get(0).

Raw types must not be used in your code, ever. The only exception to this rule is using it as an operand of the instanceof operator. Since instanceof checks for run-time type and type arguments have been erased, we can only use the instanceof operator on raw types.

Another win for information hiding! ↩