Java中数组的空指针异常

Question

The following code is returning a NullPointerException in Java. Can anyone clarify my mistake?

public void deposit2()
{
    BankAccounts[] accounts2 = new BankAccounts[10];
    accounts2[3].deposit();
}

Answer 1

BankAccounts[] accounts2 = new BankAccounts[10];

is the same as

BankAccounts[] accounts2 = {null, null, null, ... null };  // (10 times)

You need to assign values to the elements of accounts2 (or, at least to element 3) before you attempt to dereference them.

Answer 2

Just declare it as you declared it in your code and after that use for loop to assign object reference to all index.

For example:

public void deposit2()
{
    BankAccounts[] accounts2 = new BankAccounts[10];
    for(int i=0;i<10;i++)
    {
        accounts2[i] = new BankAccounts();
    }
    accounts2[3].deposite();
}

Answer 3

BankAccounts[] accounts2 = new BankAccounts[10]; creates an object ie an array of bank accounts with 10 references that can point to 10 BankAccount objects,they are initailized to null , you need to create the actual bank objects , try this

BankAccounts[] accounts2 = new BankAccounts[10];

for(BankAccounts b:accounts2)//for each loop
{
b=new BankAccounts();
}