Too sad, Xiao Wang was crying over these 10 Java interview questions. The implementation principle of Java collection

try { } catch (Exception e) {

return 0;

} finally {

System.out.println("finally returnCatchExec");

return -1;

}

}

public static void exitTryExec() {

try {

System.exit(0);

} catch (Exception e) {

} finally {

System.out.println("finally exitTryExec");

}

}

public static void exitCatchExec() {

try { } catch (Exception e) {

System.exit(0);

} finally {

System.out.println("finally exitCatchExec");

}

}

}

The result of program execution is as follows ：

finally returnTryExec

finally returnCatchExec

Third question , Private or static methods can be overridden （override） Do you ？

The reason why Xiao Wang didn't answer this question correctly , Because he's not sure about the relationship between private methods or static methods and rewriting .

The two overridden method names are the same , The number of method parameters is the same ; But a method is in the parent class , The other one is in the subclass .

class LaoWang{

public void write() {

System.out.println(" Lao Wang wrote a book 《 The count of monte cristo 》");

}

}

class XiaoWang extends LaoWang {

@Override

public void write() {

System.out.println(" Xiao Wang wrote a book 《 Camellia woman 》");

}

}

public class OverridingTest {

public static void main(String[] args) {

LaoWang wang = new XiaoWang();

wang.write();

}

}

Parent class LaoWang There is one write() Method （ No arguments ）, The method body is to write a book 《 The count of monte cristo 》; Subclass XiaoWang Overriding the write() Method （ No arguments ）, But the method body is to write a 《 Camellia woman 》.

stay main In the method , We declared a type of LaoWang The variable of wang. During compilation , The compiler will check LaoWang Class contains write() Method , Find out LaoWang Like , So compile through . During operation ,new One. XiaoWang object , And assign it to wang, here Java Virtual machines know wang The reference is XiaoWang object , So the call is subclass XiaoWang Medium write() Method instead of parent class LaoWang Medium write() Method , So the output is zero “ Xiao Wang wrote a book 《 Camellia woman 》”.

Private methods are invisible to subclasses , It is only visible in the class currently declared ,private Keywords meet the highest level of encapsulation . in addition ,Java The private methods in are bound by static binding at compile time , Independent of the object type held by a specific reference variable .

Method rewriting works for dynamic binding , So private methods cannot be overridden .

class LaoWang{

public LaoWang() {

write();

read();

}

public void write() {

System.out.println(" Lao Wang wrote a book 《 The count of monte cristo 》");

}

private void read() {

System.out.println(" Lao Wang is reading 《 Hamlet 》");

}

}

class XiaoWang extends LaoWang {

@Override

public void write() {

System.out.println(" Xiao Wang wrote a book 《 Camellia woman 》");

}

private void read() {

System.out.println(" Xiao Wang is reading 《 Merchant of Venice 》");

}

}

public class PrivateOrrideTest {

public static void main(String[] args) {

LaoWang wang = new XiaoWang();

}

}

The program output is as follows ：

Xiao Wang wrote a book 《 Camellia woman 》

Lao Wang is reading 《 Hamlet 》

In the construction method of the parent class , Respectively called write() and read() Method ,write()

The method is public Of , can

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To be rewritten , So the subclass's write() Method ,read() The method is private , Can't be rewritten , So the execution is still of the parent class read() Method .

Similar to private methods , Static methods are also bound by static binding at compile time , Independent of the object type held by a specific reference variable . Method rewriting works for dynamic binding , So static methods cannot be overridden .

public class StaticOrrideTest {

public static void main(String[] args) {

Laozi zi = new Xiaozi();

zi.write();

}

}

class Laozi{

public static void write() {

System.out.println(" Laozi wrote a book 《 The count of monte cristo 》");

}

}

class Xiaozi extends Laozi {

public static void write() {

System.out.println(" Boy wrote a book 《 Camellia woman 》");

}

}

The program output is as follows ：

Laozi wrote a book 《 The count of monte cristo 》

Reference variables zi The type of Laozi, therefore zi.write() The execution is in the parent class write() Method .

Static methods are also called class methods , You can call... Directly through the class name , When called through an object ,IDE There's a warning .

Fourth question ,1.0/0.0 What is the result ？ Will it throw an exception , There will still be compilation errors ？

The reason why Xiao Wang didn't answer this question correctly , It's because he didn't go deep into double The type and int Division of type .

The number is Java It can be divided into two kinds , One is plastic surgery , One is floating point . I'm not sure. Let's study it first [ data type ]( ).

When a floating-point number is divided by 0 When , The result is Infinity perhaps NaN.

System.out.println(1.0 / 0.0); // Infinity

System.out.println(0.0 / 0.0); // NaN

Infinity It means infinity in Chinese ,NaN This is not a number （Not a Number）.

When an integer is divided by 0 When （10 / 0）, It throws an exception ：

Exception in thread "main" java.lang.ArithmeticException: / by zero

at com.itwanger.eleven.ArithmeticOperator.main(ArithmeticOperator.java:32)

Usually , When we divide integers , You need to determine whether the divisor is 0, In case the program throws an exception .

Fifth question ,Java Support multiple inheritance ？

The reason why Xiao Wang didn't answer this question correctly , It's because he knows , Multiple inheritance can be achieved through interfaces .

To define two interfaces ,Fly Can fly ,Run Can run .

public interface Fly {

void fly();

}

public interface Run {

void run();

}

Then let a class implement both interfaces at the same time .

public class Pig implements Fly,Run{

@Override

public void fly() {

System.out.println(" Flying pigs ");

}

@Override

public void run() {

System.out.println(" Running pigs ");

}

}

But when it comes to multiple inheritance , The key words discussed are extends, Instead of implements.

Java Only single inheritance is supported , It's because of the diamond problem . If two classes inherit a parent class with a specific method , Then the method may be overridden by two subclasses . then , If you decide to inherit these two subclasses at the same time , So when you call the override method , The compiler doesn't know which subclass's method you want to call .

class C At the same time, it inherits the class A And the class B, class C Object calling class A And the class B Method overridden in , You don't know the calling class A Methods , Or class B Methods .

Sixth question , When in HashMap Put an existing key when , What's going to happen ？

The reason why Xiao Wang didn't answer this question correctly , It's because he didn't go deep into HashMap How it works .

Hash, In general, the translation is “ hash ”, There are also direct transliteration for “ Hash ” Of , What does this mean ？ It is to map any length of data to a fixed length field through an algorithm （ Hash value ）.

A little more intuitive , It's just a bunch of data wang Hybridity , Output another piece of fixed length data er—— As data wang Characteristics of . We usually use a string of fingerprints to map someone , Don't look down on fingerprints as big as your fingers , It's hard to find a second one that's the same as you in your area （ People's hash algorithm is also powerful , Is there any ）.

For any two different data blocks , It's very unlikely that their hash values will be the same , in other words , For a given block of data , It's extremely difficult to find the same block of data as its hash value . also , For a block of data , Even if only one bit of it is changed , The hash value changes will be very large —— That's exactly what it is. Hash The value of existence ！

You should know ,HashMap The underlying data structure of is an array , adopt hash() Method to determine the subscript .

static final int hash(Object key) {

int h;

return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

}

When we put in a key value pair , Will call first hash() Method pair key Hash algorithm , If key It's the same , So the hash result is the same , It means that the subscripts in the array are the same , The new value will override the original value .

Question seven , What will the following code print out ？

public class Test {

public static void main(String[] args) {

char[] chars = new char[]{'\u0097'};

String str = new String(chars);

byte[] bytes = str.getBytes();

System.out.println(Arrays.toString(bytes));

}

}

The reason why Xiao Wang didn't answer this question correctly , It's because he didn't have in-depth knowledge of character coding .

In this procedure , We create a string object from an array of characters , And then call String Class getByte() Method to get the byte array and print it to the console .

The core of this interview question is not the final printed result （ The result is uncertain ）, It's character coding . Usually , We're calling getBytes() When the method is used , To specify the encoding , for instance str.getBytes(StandardCharsets.UTF_8).

When we don't specify the encoding ,JDK Will call the platform's default character encoding , And different operating systems , The codes are not the same ,bytes And the results will be different .

When using UTF_8 when , The result is -62, -105, When using GB2312 when , The result is 63.

The eighth question , When a method is thrown in a parent class NullPointerException when , Whether you can use throw RuntimeException To rewrite it ？

The reason why Xiao Wang didn't answer this question correctly , Because he was rewritten （overriding） And overloading （overloading） I'm confused with the concept of .

Method rewriting and method overloading , Method names can be exactly the same , But the fundamental difference is that method rewriting occurs at runtime , Method overloading occurs at compile time .

in addition , The rules for method rewriting and method overloading are also different . stay Java in , Can not rewrite private、static and final Method , But you can overload them .

Let's focus on the rules for method rewriting ：

1） Method signature must be the same , Including return types 、 Number of parameters 、 The type and order of the parameters .

2） An overridden method cannot throw an exception at a higher level than in the parent class . for instance , If the method in the parent class throws IOException, Then overridden methods in subclasses cannot throw Exception, It can be IOException Or does not throw any exceptions . This rule only applies to checkable exceptions .

Author Welfare

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Give back to fans , Brimming with sincerity ！！！

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