My name
is
Jon Skeet

Field initialization in java class outside the methods' bodies

Please explain below behavior, why first statement is valid while other one is invalid and throws error.

public class Test{
    private String firstName="John";// is Valid
    //Below is invalid
    private String lastName; 
    lastname="Doe";
}

A class can only contain declarations (and static/instance initializers). A field declaration can contain an initializer, as per firstName - and your declaration of lastName is valid, but the assignment after it is just a statement, and a class can't directly contain statements.

If you want to separate declaration from assignment, you either need to put the assignment in a constructor:

public class Test {
    private String lastName;

    public Test() {
        lastName = "Doe";
    }
}

or use an instance initializer (less common in my experience):

public class Test {
    private String lastName;

    {
        lastName = "Doe";
    }
}

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