Shenzhen longhui science and technology recruitment examination questions

1. The following lines of code will not generate warnings or errors after compilation.

1)char c = " a "； 2) Byte b = 2573) Boolean b = empty; 4)int I = 10； 5) floating point f =1.3;

2. What happens when you compile the following code?

Public class MyClass {

public static void main(String arguments[]){

Method (parameter);

}

public void method(String[]arguments){

System.out.println (parameter);

system . out . println(arguments[ 1])；

}

}

1) ErrorVoid method cannot be statically referenced.

2) The wrong method is mainly incorrect.

3) Error array must contain parameters.

4) The method must be declared with a string.

3.3. The value range of byte is:

1)- 128 to127; 2)-255 to 256; 3)-65535 to 65536; 4) depending on different java virtual machines;

4. What is the output of the following command?

good morning

Public class myprog{

Public static void main(String argv[])

{

System.out.println(argv[2])

}

}

1) myprog

2) Good

3) in the morning

4) Throw an exception: "java.lang.arrayindexoutofboundsexception: 2"

5. Which of the following variables are not legal variables?

1)2 variable 2)variable 2 3)_ what available 4)_ 3 _ 5)$ another var 6)# myvar

6. What will happen after the following programs are compiled and run?

Public class MyClass{

Static int I；;

Public static void main(String argv[]){

system . out . println(I)；

}

}

1) error variable I may not be initialized 2) null 3) 1 4) 0.

7. Strings s1= new string ("Hello")

String s2 = new string ("there");

String S3 = new String()；

Which of the following is true?

1)S3 = s 1+S2； 2)S3 = s 1-S2； S3 = s 1 & amp； S2； S3 = s 1 & amp； & ampS2；

8. What will happen after the following code is compiled and run?

Abstract class MineBase {

Abstract void method ();

Static int I；;

}

Public-level mines expand mine bases (

Public static void main(String argv[]){

int[]ar = new int[5]；

for(I = 0； I & ltar. Length; i++)

system . out . println(ar[I])；

}

}

1) Print out 0 to 5 2) Error: Ar was used before initialization.

3) The error mine must be declared as abstract 4) IndexoutoBoundes error

9. Which two can be used to create a new thread? (Select two items)

1) extends java.lang.Thread and overrides the run method.

2) Extend java.lang.Runnable and cover the start method.

3) Implement java.lang.Thread and run method.

4) Implement java.lang.Runnable and run method.

5) Implement java.lang.Thread and implement the start method.

10. Write the running result of the following code.

Public class pass {

Static int j = 20

Public static void main(String argv[]){

int I = 10；

Pass p = new Pass()；

Method (1);

system . out . println(I)；

system . out . println(j)；

}

public void method(int x){

x = x * 2；

j = j * 2；

}

}

x = 10； y=40

1 1.class superclass {public float AAA () {return 3.14f; } }

Common class test extension superclass (

The floating AAA () {return 5.14f; }

Male and female AAA (double double1) {return double1; }

Public void AAA(){ }// Error

Public float AAA(float float 1){ return float1; }

}

//Overloads cannot have the same method name, but they must have different types.

12.public int bbb(){

Static int I = 0；; //Error

i++；

Return I;

}

Static variables cannot be defined in methods, so remember.

13 . public static void main(String arg[]){

Short b=57, a = 47

Double c= (short) b/a * 2.0;

int d =(short)b/a * 2；

}

Pay attention to the difference.

14. Public class test {

Public static void main(String[] args) {

String s;

Test test = newtest ();

test . f(s)； //Error

}

Public void f (string s){

s = " 1223 "；

system . out . println(s)；

}

}

Error: The local variable s may not be initialized.

1. Initialization of member variables

Instance variables can be explicitly initialized in definitions or constructors. If these two places are not initialized, the default value of automatic initialization will be retained. Variables of type final must be explicitly initialized.

Static variables can be explicitly initialized in definitions or static code blocks. If neither place is initialized, the default value of automatic initialization is retained. Static variables of type final must be explicitly initialized in the definition or in a static code block.

2. Initialization of local variables

Local variables must be explicitly initialized before use. If the compiler can confirm that local variables cannot be initialized before use, the compiler will report an error.

If the local variable is uninitialized and not used in the method, both compilation and running will pass.

Can be corrected to: string s = null.

Public course examination {

Public static void main(String[] args) {

int I = 0；

Test test = newtest ();

test . f()；

}

Public void f(){

i++； //Error

system . out . println(I)； //Error

}

}

Error: never read local variable i.

16. abstract class AAA {int I = 0；; }

Class superclass extension AAA {I = 3；; }//Error

Common class test extension superclass (

Public static void main(String[] args) {

Test test = newtest ();

test . f()；

}

Public void f(){

i++；

system . out . println(I)；

}

}

Error: Syntax error on tag' i', after this tag should be VariableDeclaratorId.

The class body is where variables and functions are declared, and you suddenly get an I = 3；; Nature can't do it. It can be used in methods.

17. Public class test {

Public static void main(String[] args) {

Test test = newtest ();

If(test. f(2)){// Error

system . out . println(" 1 1 1 ")；

} Otherwise {

system . out . println(" 222 ")；

}

}

public int f(int I){ return i++； }

}

Error: Type mismatch: Cannot convert from int to boolean.

In JAVA, if () uses boolean expressions instead of numeric values, which is different from C/C++. Boolean types and numeric types cannot be converted to each other.

In the switch(expr 1) statement, expr 1 must be compatible with int type assignment. Byte, short or type can be automatically upgraded; Floating-point or long expressions are not allowed.

In case (expr2) statement, expr2 must be a constant expression compatible with int type assignment. When the value of a variable or expression cannot match any case value, the default program code will be executed. If there is no break statement as the end sentence of the case code segment, the program will continue to execute the code block of the next case without checking the value of the next case expression.

18 public course exam (

byte[] arry 1，arry 2[]；

byte[][]arry 3；

Byte array 4 [] [];

Public void f(){

arry 1 = arry 2； //Error

arry2 = arry3

arry2 = arry4

}

}

Error: Type mismatch: Cannot convert from byte [] [] to byte [].

19. abstract class AAA {int I = 0；; }

Common class test extension AAA (

Public static void main (strinarg []) {

int[]ar = new int[5]；

for(I = 0； I & ltar. Length; I++)// Error

system . out . println(ar[I])；

}

}

Error: Non-static field i cannot be statically referenced.

Corrected to abstract class AAA {static int I = 0；; } Just do it.

public int bbb(){

int I； //Error

i++；

Return I;

}

Error: local variable I may not be initialized.

When a variable is defined in a function, java will use an error message during compilation to tell you that it has not been initialized.

Initialization of member variables:

2 1.class CtorBase {

ctor Base(int k){ system . out . println(" Base Created ")； }

}

The common class complex extends the CtorBase {

Complex (int k) {system. out.println ("complex creation"); }//Error

Public static void main (strinargs []) {

Complex a = new complex (10);

}

}

Error: Implicit super constructor, CtorBase (), is undefined. You must explicitly call another constructor.

Rule 1: The compiler always ensures that a class has at least one constructor; If the class has no constructor, the compiler will construct a parameterless constructor for it. Rule 2: If a class already has a constructor, the compiler will not construct another constructor for it, whether it is a parameterized function or a parameterless function.

In this process, the compiler will call the parameterless constructor of its parent class. If its parent class has no default (no parameters) constructor, the construction process will go wrong and the compilation will not pass.

22.class CtorBase

int I；

ctor base(int k){ this . I = k； }

}

The common class complex extends the CtorBase {

Complex number (integer) (

Super (k);

}

Public static void main (strinargs []) {

Complex a = new complex (10);

system . out . println(" a . I = "+a . I ")；

}

}

Results: a.i= 10.

23.class CtorBase {

int I；

ctor base(int k){ this . I = k； }

CtorBase() { }

}

The common class complex extends the CtorBase {

Complex number (int k) {}

Public static void main (strinargs []) {

Complex a = new complex (10);

system . out . println(" a . I = "+a . I ")；

}

}

Results: a.i=0.

24。

Common class test {

Static intcnt = 0;

Static intsum = 0;

Int number;

Public static void main (strinargs []) {

A A = new A()；

B B = new B()；

Test test 1 = new test (11);

Test test2 = new test (22);

Test2 = new test (33);

system . out . println(" CNT = "+CNT)；

system . out . println(" sum = "+sum ")；

a . show()；

b . show()；

}

Test (integer) {

Number = n；;

cnt++；

sum+= number；

}

}

Level a

int a = 2；

void show(){

system . out . println(" a 1 = "+a ")；

}

}

Class b extends A{

int a = 3；

void show(){

system . out . println(" a2 = "+super . a ")；

super . show()；

system . out . println(" a3 = "+a)；

}

}

Results:

cnt=3

Sum =66

a 1=2

a2=2

a 1=2

a3=3

25.

Grade ValHold{

public int I = 10；

}

Open class demonstration {

public void method(){

ValHold v = new ValHold()；

Another (5);

system . out . println(v . I)；

}

public void other(ValHold v){

v.i = 20

ValHold VH = new ValHold()；

v = vh

system . out . println(v . I)；

}

Public static void main(String[] argv){

Demo o = new Demo()；

o . amethod()；

}

}

Results:

10

20

When a reference variable is passed to a method as a parameter, the value of the variable can be changed in this method, that is, the object to which the reference points (vh is assigned to V in this question). However, after calling the method, the amount of change is restored to the original value, that is, the variable still points to the original object. (that is, after calling another (v), v reverts to the first val hold v = new val hold (); However, if the data (attribute) of the object pointed by the reference changes in the method, after the method call, even if the reference still points to the original object, one attribute of this object has changed (the I value of V has changed when v.i=20 is executed, so after calling another one, v.i becomes 20).

26.

Common class test {

Public static void main(String [] args){

Base b = new subclass ();

system . out . println(b . x)；

system . out . println(b . method())；

}

}

Category basis {

int x = 2；

Int method () {

Returns x;

}

}

Class subclasses extend Base{

int x = 3；

Int method () {

Returns x;

}

}

Results: 2 3

After the variable b is declared, the part of the object you can access is only the part of Base; Special parts of subclasses are hidden. This is because the compiler should realize that e is a base, not a subclass, except for rewriting methods. Abstract and final are mutually exclusive, the former is dedicated to inheritance, and the latter prohibits inheritance; Abstract methods in abstract classes cannot be static, and static methods cannot be rewritten, they are just hidden.

27。

Common class test {

Public static string setfiletype (stringfname) (

int p=fname.indexOf(' . ');

If (p & gt0) fname=fname.substring(0, p);

fname+="。 TXT "；

Return fname

}

Public static void main (strinargs []) {

String theFile = " Program.java

string s = set filetype(the file)；

system . out . println(" Created "+the file)；

system . out . println(" Created "+s)；

}

}

String becomes a safe String in java, and any operation on String is to regenerate a copy of string first, and then operate this copy. Therefore, String can be regarded as the transmission of value when passing parameters. In other words, if you need to modify a string and return the modified string, you must get the return value again.