J2se exam notes summarize some areas that are easy to ignore

1. Run PROBA 4. What is the result of the following code?

Public class PROBA {<br/> Public static void main (string [] ARGs) {<br/> final int num1 = 0; <br/> final int num2 = integer. parseint (ARGs [0]); <br/> assert num1> num2: "The value must be negative"; <br/> system. out. println ("You specified" + num2); <br/>}< br/>}

 

 

The input result is you specified 4.

 

Reason: assertions is disabled by default.

 

2. Will the following code run?

 

int [] array = new int [Integer.MAX_VALUE + 1];

 

Answer: Yes, but negativearraysizeexception may occur, because the value range of the Java array index is [147,483,647,], while that of integer. max_value + 1 is negative.

 

3. Can a started thread be started again?

 

Answer: No. Start () can only be used once. Otherwise, an illegalthreadstartexception exception will be thrown.

 

4. Test A for execution mechanism. What is the result of the following code input?

 

Public class test {<br/> static Boolean Foo (char c) {<br/> system. out. print (c); <br/> return true; <br/>}< br/> Public static void main (string [] argv) {<br/> int I = 0; <br/> for (FOO ('A'); Foo ('B') & (I <2 ); foo ('C') {<br/> I ++; <br/> Foo ('D '); <br/>}< br/>} 

 

Answer:

 

Abdcbdcb, most of the errors are due to not careful consideration.

 

5. What is the problem with the following code?

 

Class test implements runnable {<br/> int I = 0; <br/> Public int run () {<br/> while (true) {<br/> I ++; <br/> system. out. println ("I =" + I); <br/>}< br/> return 1; <br/>}< br/>} 

 

A: The return type of run should be void ..

 

5. Compare the values. What is the output result of the following code?

 

Public class G6 {<br/> Public static void main (string [] ARGs) {<br/> double D = 0.0, d1 =-0.0; </P> <p> If (D = D1) {<br/> system. out. println (true); <br/>}else {<br/> system. out. println (false); <br/>}</P> <p> If (0.0 =-0.0) {<br/> system. out. println (true); <br/>}else {<br/> system. out. println (false); <br/>}</P> <p> 17 :}< br/> 18 :} 

 

A: True, true

 

6. & bit operations

 

Int A = 128;

Int B = 255;

Int C;

C = A & B

 

What is C?

Answer: 128.

  00000000 00000000 00000000 10000000

  00000000 00000000 00000000 11111111

= 00000000 00000000 00000000 10000000

 

 

7. Nearby matching

 

What is the output result of the following code?

 

Public class test {<br/> Public static void method (Object O) {<br/> system. out. println ("Object:" + O); <br/>}</P> <p> Public static void method (string s) {<br/> system. out. println ("string:" + S); <br/>}</P> <p> Public static void main (string [] ARGs) {<br/> method (null); <br/>}< br/>} 

 

A: The result is string: null.

String is the subclass of the object. It has a priority over NULL for the object. If the method (null) is changed to method (123), the result will be object: 123.

 

8. null

 

Which of the following statements is true?

 

Class

boolean b = null instanceof A

 

A: Yes. null has no class.

 

9 can the following code run normally?

 

Public class test {<br/> Public static void main (string [] ARGs) {<br/> char B = 127; </P> <p> while (false) {<br/> system. out. println (B); <br/>}< br/>} 

 

A: No. Compilation error. system. Out. println (B); cannot be executed.