Java Cast and Convert Types

Casts. Data is typed.

Often its type is considered part of the data: it is metadata, information about itself. Casting, in converting types, thus changes data.

Casting and converting in Java is a practical operation. We use implicit (hidden) and explicit (specified) casts. Complex conversions too are needed.

Object and String. This program casts a String variable to its ancestor class Object (all classes derive from Object). It then casts the object back to a String.

Implicit: An implicit cast uses no special expression syntax. Rather it converts one type to another—the compiler decides on its own.

Explicit: This is specified directly in the syntax. The Object is explicitly cast to a String in the third statement.

Based on:

Java 8

Java program that uses String, Object cast

public class Program {
    public static void main(String[] args) {

	String value = "cat";
	Object temp = value; // Implicitly cast to Object.
	String value2 = (String) temp; // Cast to String.

	System.out.println(value2);
    }
}

Output

cat

Numeric. Numbers can be cast with the same syntax as classes. For casting to a larger byte size number (a "widening" cast) no cast is required.

But: For "narrowing" casts, where a larger byte size number is reduced to fit in a smaller type, a cast is required.

Data loss: This may occur in a narrowing conversion. Be sure the value can be represented in the new data size.

Java program that uses numeric casts

public class Program {
    public static void main(String[] args) {

	int size = 5;
	double size2 = size; // No cast needed.
	byte size3 = (byte) size; // A larger type must be cast down.

	System.out.println(size);
	System.out.println(size2);
	System.out.println(size3);
    }
}

Output

5
5.0
5

Invalid cast, ClassCastException. Some casts will fail. And often the compiler will detect an invalid cast at compile-time, saving us the trouble of running an invalid program.

Here: The Object cannot be cast to String, because it is of type StringBuilder. A ClassCastException occurs.

Java program that uses invalid cast

public class Program {
    public static void main(String[] args) {

	StringBuilder builder = new StringBuilder();
	builder.append("cat");

	Object temp = builder; // This is fine.
	String value = (String) temp; // This causes an error.
	System.out.println(value);
    }
}

Output

Exception in thread "main" java.lang.ClassCastException:
    java.lang.StringBuilder cannot be cast to java.lang.String
    at program.Program.main(Program.java:10)

Array covariance. This concept means an array of a derived class can be cast to an ancestor class array. So an array of Strings can be cast to an array of Objects.

Object Arrays

Tip: Array covariance is most often useful when we want to pass an array of any type as an Object array to a commonly-used method.

And: In most programs, array covariance is not important. Using exact types is clearer and faster.

Java program that uses Object array

public class Program {
    public static void main(String[] args) {

	String[] values = { "cat", "dog", "rabbit" };
	// A String array is also an Object array.
	Object[] array = values;

	// Loop over objects and print String lengths.
	for (Object v : array) {
	    System.out.println(((String) v).length());
	}
    }
}

Output

3
3
6

Casts versus conversions. Typically casts describe special syntax instructions to convert types. Conversions meanwhile include more complex operations, like converting a String to an array.

Number to String. A number cannot be cast to a String—instead we must convert it with a method like Integer.toString. This method returns a String version of the int argument.

Int

Java program that converts number to String

public class Program {
    public static void main(String[] args) {

	int number = 100;
	String result = Integer.toString(number); // Number to String.
	System.out.println(result);
    }
}

Output

100

String to number. We convert a String to an integer by parsing it. The Integer.parseInt method can do this. It receives a String argument and returns an int.

Strings

Java program that converts String to number

public class Program {
    public static void main(String[] args) {

	String value = "100";
	int result = Integer.parseInt(value); // String to number.
	System.out.println(result);
    }
}

Output

100

Boolean to int. Sometimes we want to convert a boolean into an int. Typically we want true to equal 1, and false to be 0. This is done with a simple method and a ternary expression.

Convert boolean to int

Casting is complex. It is often language-specific, so we must memorize its syntax. Conversions require more steps. They are often reused in many projects and stored in utility classes.

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