What does it mean to continue the class declaration inside class constructor?

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I found this code and I want to understand why the programmer did this instead of continuing the declaration in the same line?

Java

public class Program {
    private Map<Integer, String> abc1;
    private Map<Integer, List<String>> abc2;
    private Function main;
    private StringBuilder Code1;

    public Program() {
        code 1 = new StringBuilder();
        abc1 = new HashMap<>();
        abc2 = new TreeMap<>();
    }
}

What I have tried:

I tried this

Java

private Map<Integer, String> new HashMap<>() ;

but they are work fine, is there any difference between the two ways?

Thank you in advance

Posted 18-Feb-20 3:07am

Hisham-Developer

Updated 18-Feb-20 3:22am

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Maciej Los 18-Feb-20 9:15am

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phil.o · Accepted Answer · 2020-02-18T03:23:00

The only difference is that initializing fields upon declaration, as in

C#

private Map<Integer, String> abc1 = new HashMap<>();

makes things a bit easier (this is a personal opinion which could be highly argued since it mostly depends on the context). Whatever constructor you will call, you are sure that these fields will be properly initialized.

Whereas, when you initialize those fields inside a constructor, then you must make sure that the fields are properly initialized while using other constructors, too.

C#

// Declaration without initialization
private Map<Integer, String> abc1;

// Default initializer
public Program()
{
   abc1 = new Map<Integer, String>();
}

// Initialize from parameter
public Program(Map<Integer, String> map)
{
   abc1 = map ?? new Map<Integer, String>();
}

// Initialize by calling default constructor
public Program(double someOtherField) : this()
{
   // ...
}