What is the default initialization of an array in Java

Question

So I m declaring and initializing an int array   static final int UN   0  int   arr   new int size   for  int i   0  i  lt  size  i          arr i    UN      Say I do this instead      int   arr   new int 5   System out println arr 0          0 will print to standard out  Also  if I do this   static final int UN   0  int   arr   new int 5   System out println arr 0   UN         true will print to standard out  So how is Java initializing my array by default  Is it safe to assume that the default initialization is setting the array indices to 0 which would mean I don t have to loop through the array and initialize it    Thanks

User · Answer

According to java      Data Type          -         Default values      byte                    -          0      short                 -        0      int                     -          0      long                    -          0L      float                  -           0 0f      double             -                   0 0d      char                      -          u0000       String  or any object     -    null      boolean                 -              false

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From the Java Language Specification         Each class variable  instance variable  or array component is initialized with a default value when it is created    15 9    15 10           For type byte  the default value is zero  that is  the value of  byte 0    For type short  the default value is zero  that is  the value of  short 0    For type int  the default value is zero  that is  0    For type long  the default value is zero  that is  0L    For type float  the default value is positive zero  that is  0 0f    For type double  the default value is positive zero  that is  0 0d    For type char  the default value is the null character  that is    u0000     For type boolean  the default value is false    For all reference types    4 3   the default value is null

User · Answer

Everything in a Java program not explicitly set to something by the programmer  is initialized to a zero value      For references  anything that holds an object  that is null  For int short byte long that is a 0  For float double that is a 0 0 For booleans that is a false  For char that is the null character   u0000   whose decimal equivalent is 0     When you create an array of something  all entries are also zeroed   So your array contains five zeros right after it is created by new   Note  based on comments    The Java Virtual Machine is not required to zero out the underlying memory when allocating local variables  this allows efficient stack operations if needed  so to avoid random values the Java Language Specification requires local variables to be initialized

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Thorbj  rn Ravn Andersen answered for most of the data types  Since there was a heated discussion about array   Quoting from the jls spec http   docs oracle com javase specs jls se7 html jls-4 html jls-4 12 5  array component is initialized with a default value when it is created   I think irrespective of whether array is local or instance or class variable it will with default values

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JLS clearly says  An array initializer creates an array and provides initial values for all its components    and this is irrespective of whether the array is an instance variable or local variable or class variable   Default values for primitive types   docs  For objects default values is null

User · Answer

Every class in Java have a constructor   a constructor is a method which is called when a new object is created  which initializes the fields of the class variables    So when you are creating an instance of the class  constructor method is called while creating the object and all the data values are initialized at that time   For object of integer array type all values in the array are initialized to 0 zero  in the constructor method  Similarly for object of boolean array  all values are initialized to false    So Java is initializing the array by running its constructor method while creating the object

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Java says that the default length of a JAVA array at the time of initialization will be 10   private static final int DEFAULT CAPACITY   10    But the size   method returns the number of inserted elements in the array  and since at the time of initialization  if you have not inserted any element in the array  it will return zero   private int size   public boolean add E e        ensureCapacityInternal size   1       Increments modCount       elementData size      e      return true     public void add int index  E element        rangeCheckForAdd index       ensureCapacityInternal size   1       Increments modCount       System arraycopy elementData  index  elementData  index   1 size - index       elementData index    element      size

[java] What is the default initialization of an array in Java?

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