[OCAJP]印出null

Given:

public class Painting {

    private String type;

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public static void main(String[] args) {
        Painting obj1 = new Painting();
        Painting obj2 = new Painting();
        obj1.setType(null);
        obj2.setType("Fresco");
        System.out.print(obj1.getType() + " : " + obj2.getType());
    }
}

What is the result?

A.

B.

C.

D. A NullPointerException is thrown at runtime

題解

使用「+」運算子時，若其中至少一個運算元為字串(物件)型態，則為字串連接運算。若連接的字串(物件)參考是null的話，則會被當作長度為4的「null」字串。