看程式碼說結果

public class Dervied extends Base {
private String name = "dervied";
public Dervied() {
tellName();
printName();
}
public void tellName() {
System.out.println("Dervied tell name: " + name);
}
public void printName() {
System.out.println("Dervied print name: " + name);
}
public static void main(String[] args){
new Dervied();
}
}
class Base {
private String name = "base";
public Base() {
tellName();
printName();
}
public void tellName() {
System.out.println("Base tell name: " + name);
}
public void printName() {
System.out.println("Base print name: " + name);
}
}

執行結果:
Dervied tell name: null
Dervied print name: null
Dervied tell name: dervied
Dervied print name: dervied

為什麼會這樣呢
並且無論修改Base的name為public還是protected,結果都是這樣.
甚至程式碼改成這樣,還是這個結果(Base類的name欄位改成了basename)

public class Dervied extends Base {
private String name = "dervied";
public Dervied() {
tellName();
printName();
}
public void tellName() {
System.out.println("Dervied tell name: " + name);
}
public void printName() {
System.out.println("Dervied print name: " + name);
}
public static void main(String[] args){
new Dervied();
}
}
class Base {
public String basename = "base";
public Base() {
tellName();
printName();
}
public void tellName() {
System.out.println("Base tell name: " + basename);
}
public void printName() {
System.out.println("Base print name: " + basename);
}
}

後臺一個大神一語道破
Base的建構函式中,呼叫的兩個方法tellName和printName,因為繼承的原因,應該執行的是子類的方法,而子類此時還沒有初始化這個資料成員,所以顯示為null.

JAVA繼承的初始化問題

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