在 Java 中创建对象数组

Question

I am new to Java and for the time created an array of objects in Java.

I have a class A for example -

A[] arr = new A[4];

But this is only creating pointers (references) to A and not 4 objects. Is this correct? I see that when I try to access functions/variables in the objects created I get a null pointer exception. To be able to manipulate/access the objects I had to do this:

A[] arr = new A[4];
for (int i = 0; i < 4; i++) {
    arr[i] = new A();
}

Is this correct or am I doing something wrong? If this is correct its really odd.

EDIT: I find this odd because in C++ you just say new A[4] and it creates the four objects.

Answer 1

This is correct.

A[] a = new A[4];

...creates 4 A references, similar to doing this:

A a1;
A a2;
A a3;
A a4;

Now you couldn't do a1.someMethod() without allocating a1 like this:

a1 = new A();

Similarly, with the array you need to do this:

a[0] = new A();

...before using it.

Answer 2

This is correct. You can also do :

A[] a = new A[] { new A("args"), new A("other args"), .. };

This syntax can also be used to create and initialize an array anywhere, such as in a method argument:

someMethod( new A[] { new A("args"), new A("other args"), . . } )

Answer 3

Yes, it creates only references, which are set to their default value null. That is why you get a NullPointerException You need to create objects separately and assign the reference. There are 3 steps to create arrays in Java -

Declaration – In this step, we specify the data type and the dimensions of the array that we are going to create. But remember, we don't mention the sizes of dimensions yet. They are left empty.

Instantiation – In this step, we create the array, or allocate memory for the array, using the new keyword. It is in this step that we mention the sizes of the array dimensions.

Initialization – The array is always initialized to the data type's default value. But we can make our own initializations.

Declaring Arrays In Java

This is how we declare a one-dimensional array in Java –

 int[] array; int array[];

Oracle recommends that you use the former syntax for declaring arrays. Here are some other examples of legal declarations –

 // One Dimensional Arrays int[] intArray; // Good double[] doubleArray; // One Dimensional Arrays byte byteArray[]; // Ugly! long longArray[]; // Two Dimensional Arrays int[][] int2DArray; // Good double[][] double2DArray; // Two Dimensional Arrays byte[] byte2DArray[]; // Ugly long[] long2DArray[];

And these are some examples of illegal declarations –

 int[5] intArray; // Don't mention size! double{} doubleArray; // Square Brackets please!

Instantiation

This is how we “instantiate”, or allocate memory for an array –

 int[] array = new int[5];

When the JVM encounters the new keyword, it understands that it must allocate memory for something. And by specifying int[5] , we mean that we want an array of int s, of size 5. So, the JVM creates the memory and assigns the reference of the newly allocated memory to array which a “reference” of type int[]

Initialization

Using a Loop – Using a for loop to initialize elements of an array is the most common way to get the array going. There's no need to run a for loop if you are going to assign the default value itself, because JVM does it for you.

All in One..! – We can Declare, Instantiate and Initialize our array in one go. Here's the syntax –

 int[] arr = {1, 2, 3, 4, 5};

Here, we don't mention the size, because JVM can see that we are giving 5 values.

So, until we instantiate the references remain null. I hope my answer has helped you..! :)

Source - Arrays in Java

Answer 4

Here is the clear example of creating array of 10 employee objects, with a constructor that takes parameter:

public class MainClass
{  
    public static void main(String args[])
    {
        System.out.println("Hello, World!");
        //step1 : first create array of 10 elements that holds object addresses.
        Emp[] employees = new Emp[10];
        //step2 : now create objects in a loop.
        for(int i=0; i<employees.length; i++){
            employees[i] = new Emp(i+1);//this will call constructor.
        }
    }
}

class Emp{
    int eno;
    public Emp(int no){
        eno = no;
        System.out.println("emp constructor called..eno is.."+eno);
    }
}

Answer 5

You are correct. Aside from that if we want to create array of specific size filled with elements provided by some "factory", since Java 8 (which introduces stream API ) we can use this one-liner:

A[] a = Stream.generate(() -> new A()).limit(4).toArray(A[]::new);

• Stream.generate(() -> new A()) is like factory for separate A elements created in a way described by lambda, () -> new A() which is implementation of Supplier<A> - it describe how each new A instances should be created.
• limit(4) sets amount of elements which stream will generate
• toArray(A[]::new) (can also be rewritten as toArray(size -> new A[size]) ) - it lets us decide/describe type of array which should be returned.

For some primitive types you can use DoubleStream , IntStream , LongStream which additionally provide generators like range rangeClosed and few others.

Answer 6

The genaral form to declare a new array in java is as follows:

type arrayName[] = new type[numberOfElements];

Where type is a primitive type or Object. numberOfElements is the number of elements you will store into the array and this value can't change because Java does not support dynamic arrays (if you need a flexible and dynamic structure for holding objects you may want to use some of the Java collections).

Lets initialize an array to store the salaries of all employees in a small company of 5 people:

int salaries[] = new int[5];

The type of the array (in this case int ) applies to all values in the array. You can not mix types in one array.

Now that we have our salaries array initialized we want to put some values into it. We can do this either during the initialization like this:

int salaries[] = {50000, 75340, 110500, 98270, 39400};

Or to do it at a later point like this:

salaries[0] = 50000;
salaries[1] = 75340;
salaries[2] = 110500;
salaries[3] = 98270;
salaries[4] = 39400;

More visual example of array creation:

To learn more about Arrays, check out the guide .

Answer 7

Yes it is correct in Java there are several steps to make an array of objects:

1. Declaring and then Instantiating (Create memory to store '4' objects):

    A[ ] arr = new A[4];

2. Initializing the Objects (In this case you can Initialize 4 objects of class A)

    arr[0] = new A(); arr[1] = new A(); arr[2] = new A(); arr[3] = new A();

   or

   for( int i=0; i<4; i++ ) arr[i] = new A();

Now you can start calling existing methods from the objects you just made etc.

For example:

  int x = arr[1].getNumber();

or

  arr[1].setNumber(x);

Answer 8

For generic class it is necessary to create a wrapper class. For Example:

Set<String>[] sets = new HashSet<>[10]

results in: "Cannot create a generic array"

Use instead:

        class SetOfS{public Set<String> set = new HashSet<>();}
        SetOfS[] sets = new SetOfS[10];  

Answer 9

Suppose the class A is such:

class A{
int rollno;
int DOB;
}        

and you want to create an array of the objects for the class A. So you do like this,

    A[] arr = new A[4];    //Statement 1
    for (int i = 0; i < 4; i++) {
    arr[i] = new A();      //Statement 2
    }

which is absolutely correct.

Here A is the class and in Statement 1 Class A is a datatype of the array. When this statement gets executed because of the new keyword an object is created and dynamically memory is allocated to it which will be equal to the space required for the 4 blocks of datatype A ie, ( for one block in the array space required is 8 bytes (4+4), I am assuming int takes 4 bytes of space. therefore total space allocated is 4*4 bytes for the array ). Then the reference of the object is given to the arr variable. Here important point to note is that Statement 1 has nothing to do with creating an object for class A ,no object is created for this class it is only used as a datatype which gives the size of the class A required for the memory allocation of the array.

Then when for loop is run and Statement 2 is executed JVM now allocates the memory for the Class A (ie creates an object) and gives its reference to the arr[i]. Every time the loop is called an object is created and the reference of it is given to arr[i].

Thus, arr[0] which holds a space of 8 bytes is given the reference of the object of the Class A and everytime loop is run new object is created and reference is given to that object so that it can now access the data in that object .