Java 101: Foundations

Java 101: Class and object initialization in Java

Java 101: Class and object initialization
Credit: Windell Oskay/Flickr

Prepare Java classes and objects for successful execution

Classes and objects in Java must be initialized before they are used. You previously learned that class fields are initialized to default values when classes are loaded and that objects are initialized via constructors, but there is more to initialization. This final installment in the Java 101: Foundations series introduces all of Java's features for initializing classes and objects.

Initializing classes

Before we explore Java's support for class initialization, let's recap the basics of Java initialization. Consider Listing 1.

Listing 1. Initializing class fields to default values


class SomeClass
{
   static boolean b;
   static byte by;
   static char c;
   static double d;
   static float f;
   static int i;
   static long l;
   static short s;
   static String st;
}

Listing 1 declares class SomeClass. This class declares nine fields of types boolean, byte, char, double, float, int, long, short, and String. When SomeClass is loaded, each field's bits are set to zero, which you interpret as follows:


false
0
\u0000
0.0
0.0
0
0
0
null

The previous class fields were implicitly initialized to zero. However, you can also explicitly initialize class fields by directly assigning values to them, as shown in Listing 2.

Listing 2. Initializing class fields to explicit values


class SomeClass
{
   static boolean b = true;
   static byte by = 1;
   static char c = 'A';
   static double d = 2.0;
   static float f = 3.0f;
   static int i = 4;
   static long l = 5000000000L;
   static short s = 20000;
   static String st = "abc";
}

Each assignment's value must be type-compatible with the class field's type. Each variable stores the value directly, with the exception of st. Variable st stores a reference to a String object that contains abc.

Referencing previously declared and subsequently declared class fields

When initializing a class field, it's legal to initialize it to the value of a previously initialized class field. For example, Listing 3 initializes y to x's value. Both fields are initialized to 2.

Listing 3. Referencing a previously declared field


class SomeClass
{
   static int x = 2;
   static int y = x;

   public static void main(String[] args)
   {
      System.out.println(x);
      System.out.println(y);
   }
}

However, the reverse is not legal: you cannot initialize a class field to the value of a subsequently declared class field. The Java compiler outputs illegal forward reference when it encounters this scenario. Consider Listing 4.

Listing 4. Attempting to reference a subsequently declared field


class SomeClass
{
   static int x = y;
   static int y = 2;

   public static void main(String[] args)
   {
      System.out.println(x);
      System.out.println(y);
   }
}

The compiler will report illegal forward reference when it encounters static int x = y;. This is because source code is compiled from the top down, and the compiler hasn't yet seen y. (It would also output this message if y wasn't explicitly initialized.)

Introducing class initialization blocks

You might want to perform complex class-based initialization after a class has been loaded and before any objects are created from that class (assuming that the class isn't a utility class). You can use a class initialization block for this task.

A class initialization block is a block of statements preceded by the static keyword that's introduced into the class's body. When the class loads, these statements are executed. Consider Listing 5.

Listing 5. Initializing arrays of sine and cosine values


class Graphics
{
   static double[] sines, cosines;
   static
   {
      sines = new double[360];
      cosines = new double[360];
      for (int i = 0; i < sines.length; i++)
      {
         sines[i] = Math.sin(Math.toRadians(i));
         cosines[i] = Math.cos(Math.toRadians(i));
      }
   }
}

Listing 5 declares a Graphics class that declares sines and cosines array variables. It also declares a class initialization block that creates 360-element arrays whose references are assigned to sines and cosines. It then uses a for statement to initialize these array elements to the appropriate sine and cosine values, by calling the Math class's sin() and cos() methods. (Math is part of Java's standard class library. I'll discuss this class and these methods in a future article.)

Combining class field initializers and class initialization blocks

You can combine multiple class field initializers and class initialization blocks in an application. Listing 6 provides an example.

Listing 6. Performing class initialization in top-down order

class MCFICIB
{
   static int x = 10;

   static double temp = 98.6;

   static
   {
      System.out.println("x = " + x);
      temp = (temp - 32) * 5.0/9.0; // convert to Celsius
      System.out.println("temp = " + temp);
   }

   static int y = x + 5;

   static
   {
      System.out.println("y = " + y);
   }

   public static void main(String[] args)
   {
   }
}

Listing 6 declares and initializes a pair of class fields (x and y), and declares a pair of static initializers. Compile this listing as shown:

javac MCFICIB.java

Then run the resulting application:

java MCFICIB

You should observe the following output:


x = 10
temp = 37.0
y = 15

This output reveals that class initialization is performed in top-down order.

<clinit>() methods

When compiling class initializers and class initialization blocks, the Java compiler stores the compiled bytecode (in top-down order) in a special method named <clinit>(). The angle brackets prevent a name conflict: you cannot declare a <clinit>() method in source code because the < and > characters are illegal in an identifier context.

After loading a class, the JVM calls this method before calling main() (when main() is present).

Let's take a look inside MCFICIB.class. The following partial disassembly reveals the stored information for the x, temp, and y fields:


Field #1

00000290        Access Flags                          ACC_STATIC
00000292        Name                                  x
00000294        Descriptor                            I
00000296        Attributes Count                      0

Field #2

00000298        Access Flags                          ACC_STATIC
0000029a        Name                                  temp
0000029c        Descriptor                            D
0000029e        Attributes Count                      0

Field #3

000002a0        Access Flags                          ACC_STATIC
000002a2        Name                                  y
000002a4        Descriptor                            I
000002a6        Attributes Count                      0

The Descriptor line identifies the JVM's type descriptor for the field. The type is represented by a single letter: I for int and D for double.

The following partial disassembly reveals the bytecode instruction sequence for the <clinit>() method. Each line starts with a decimal number that identifies the zero-based offset address of the subsequent instruction:

  
  0        bipush 10
  2        putstatic MCFICIB/x I
  5        ldc2_w #98.6
  8        putstatic MCFICIB/temp D
 11        getstatic java/lang/System/out Ljava/io/PrintStream;
 14        new java/lang/StringBuilder
 17        dup
 18        invokespecial java/lang/StringBuilder/<init>()V
 21        ldc "x = "
 23        invokevirtual
java/lang/StringBuilder/append(Ljava/lang/String;)Ljava/lang/StringBuilder;
 26        getstatic MCFICIB/x I
 29        invokevirtual
java/lang/StringBuilder/append(I)Ljava/lang/StringBuilder;
 32        invokevirtual
java/lang/StringBuilder/toString()Ljava/lang/String;
 35        invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
 38        getstatic MCFICIB/temp D
 41        ldc2_w #32
 44        dsub
 45        ldc2_w #5
 48        dmul
 49        ldc2_w #9
 52        ddiv
 53        putstatic MCFICIB/temp D
 56        getstatic java/lang/System/out Ljava/io/PrintStream;
 59        new java/lang/StringBuilder
 62        dup
 63        invokespecial java/lang/StringBuilder/<init>()V
 66        ldc "temp = "
 68        invokevirtual
java/lang/StringBuilder/append(Ljava/lang/String;)Ljava/lang/StringBuilder;
 71        getstatic MCFICIB/temp D
 74        invokevirtual
java/lang/StringBuilder/append(D)Ljava/lang/StringBuilder;
 77        invokevirtual
java/lang/StringBuilder/toString()Ljava/lang/String;
 80        invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
 83        getstatic MCFICIB/x I
 86        iconst_5
 87        iadd
 88        putstatic MCFICIB/y I
 91        getstatic java/lang/System/out Ljava/io/PrintStream;
 94        new java/lang/StringBuilder
 97        dup
 98        invokespecial java/lang/StringBuilder/<init>()V
101        ldc "y = "
103        invokevirtual
java/lang/StringBuilder/append(Ljava/lang/String;)Ljava/lang/StringBuilder;
106        getstatic MCFICIB/y I
109        invokevirtual
java/lang/StringBuilder/append(I)Ljava/lang/StringBuilder;
112        invokevirtual
java/lang/StringBuilder/toString()Ljava/lang/String;
115        invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
118        return

The instruction sequence from offset 0 through offset 2 is equivalent to the following class field initializer:

static int x = 10;

The instruction sequence from offset 5 through offset 8 is equivalent to the following class field initializer:

static double temp = 98.6;

The instruction sequence from offset 11 through offset 80 is equivalent to the following class initialization block:


static
{
   System.out.println("x = " + x);
   temp = (temp - 32) * 5.0/9.0; // convert to Celsius
   System.out.println("temp = " + temp);
}

The instruction sequence from offset 83 through offset 88 is equivalent to the following class field initializer:

static int y = x + 5;

The instruction sequence from offset 91 through offset 115 is equivalent to the following class initialization block:


static
{
   System.out.println("y = " + y);
}

Finally, the return instruction at offset 118 returns execution from <clinit>() to that part of the JVM that called this method.

Initializing objects

After a class has been loaded and initialized, you'll often want to create objects from the class. As you learned in my recent introduction to programming with classes and objects, you initialize an object via the code that you place in a class's constructor. Consider Listing 7.

Listing 7. Initializing an object via a constructor


class City
{
   private String name;
   int population;

   City(String name, int population)
   {
      this.name = name;
      this.population = population;
   }

   @Override
   public String toString()
   {
      return name + ": " + population;
   }

   public static void main(String[] args)
   {
      City newYork = new City("New York", 8491079);
      System.out.println(newYork); // Output: New York: 8491079
   }
}

Listing 7 declares a City class with name and population fields. When a City object is created, the City(String name, int population) constructor is called to initialize these fields to the called constructor's arguments. (I've also overridden Object's public String toString() method to conveniently return the city name and population value as a string. System.out.println() ultimately calls this method to return the object's string representation, which it outputs.)

Before the constructor is called, what values do name and population contain? You can find out by inserting System.out.println(this.name); System.out.println(this.population); at the start of the constructor. After compiling the source code (javac City.java) and running the application (java City), you would observe null for name and 0 for population. The new operator zeroes an object's object (instance) fields before executing a constructor.

As with class fields, you can explicitly initialize object fields. For example, you could specify String name = "New York"; or int population = 8491079;. However, there's usually nothing to gain by doing this, because these fields will be initialized in the constructor. The only benefit that I can think of is to assign a default value to an object field; this value is used when you call a constructor that doesn't initialize the field:


int numDoors = 4; // default value assigned to numDoors

Car(String make, String model, int year)
{
   this(make, model, year, numDoors);
}

Car(String make, String model, int year, int numDoors)
{
   this.make = make;
   this.model = model;
   this.year = year;
   this.numDoors = numDoors;
}

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