return “sub”;
}
public static void main(String args[]){
J_Base app=new J_SubClass();
//插入需要的语句
} }
A System.out.println(app.mb_getString());
B System.out.println((J_Base)app.mb_getString());
C System.out.println((J_SubClass)app.mb_getString());
D System.out.println(((J_SubClass)app).mb_getString());
26.下面程序的正确结论是
class J_Base{ }
class J_SubClass extends J_Base{
public String mb_getString(){
return “sub”;
}
public static void main(String args[]){
J_SubClass app=(J_SubClass)new J_Base();
System.out.println(((J_SubClass)app).mb_getString());
} }
A 发生编译错误
B 能编译但不能运行
C 能编译能运行，输出sub
D ((J_SubClass)app).mb_getString() 是不正确的语法
27.下面程序的输出结果是
class J_Base{
public int m_data=5;
public void mb_print(){
System.out.print(m_data);
} }
class J_SubClass extends J_Base{
public int m_data=2;
public void mb_print(){
System.out.print(m_data);
}
public static void main(String args[]){
J_Base app=new J_SubClass();
app.mb_print();
System.out.println(app.m_data);
} }
A 22 B 55 C 25 D 52
28.下面哪些方法置于指定位置会导致编译错误
public class J_Super {
public float getNum(){
return 3.0f;
} }
public class J_Sub extends J_Super{
//指定位置
}
A public float getNum(){return 4.0f;}
B public void getNum(){}
C public void getNum(double d){}
D public double getNum(float d){return 4.0d; }
29.已知字母a和b对应的数值分别是97和98,关于下面程序哪些选项是正确
public class J_Test {
static char m_name;
static int m_age;
public static void mb_setData(char n){
m_name=n;
}
public static void mb_setData(int age){
m_age=age;
}
public static void main(String[] args) {
mb_setData((int)’a');
mb_setData((char)98);
System.out.println(m_name+” “+m_age);
} }
A 结果输出 a 98 B 结果输出 b 97
C 结果输出 b 98 D 结果输出a 97
30.下面程序哪些结论是正确的
public class J_Test {
static short m_index;
static int m_age;
public static void mb_setData(short n){
m_index=n;
}
public static void mb_setData(int age){
m_age=age;
}
public static void main(String[] args) {
mb_setData(98);
System.out.println(m_index+” “+m_age);
} }
A 结果输出 98 0 B结果输出0 98
C 结果输出 0 0 D 结果输出98 98
E 程序编译错误
31.下面程序哪些结论是正确的
public class J_Test {
static short m_index;
static int m_age;
public static void mb_setData(short n){
m_index=n;
}
public static void main(String[] args) {
mb_setData(98);
System.out.println(m_index+” “+m_age);
} }
A 结果输出 98 0 B结果输出0 98
C 结果输出 0 0 D 结果输出98 98
E 程序编译错误