核心Java面试题解答

package com.walterlife.blog;

public class ThreadInvokeStartTwice extends Thread {

    @Override
        public void run() {
            // TODO Auto-generated method stub
            System.out.print("Thread runing" + Thread.activeCount());
        }

    public static void main(String[] args) {
        ThreadInvokeStartTwice thread = new ThreadInvokeStartTwice();
        thread.start();
        thread.start();
    }
}
// output
    Thread runing2Exception in thread "main" java.lang.IllegalThreadStateException
    at java.lang.Thread.start(Unknown Source)
at com.walterlife.blog.ThreadInvokeStartTwice.main(ThreadInvokeStartTwice.java:14)

package com.walterlife.blog;

import java.util.ArrayList;
import java.util.List;

class InsensitiveSearch {
    private List<String> searchColl;

    public InsensitiveSearch(List search) {
        this.searchColl = search;
    }

    public boolean contains(String str) {
        for (String s : searchColl) {
            if (str.equalsIgnoreCase(s)) {
                return true;
            }
        }
        return false;
    }

    public int indexOf(String str) {
        for (int index = 0; index < searchColl.size(); index++) {
            if (str.equalsIgnoreCase(searchColl.get(index))) {
                return index;
            }
        }
        return -1;
    }
}

public class SearchInArrayList {

    public static void main(String[] args) {

        List<String> list = new ArrayList<>();
        list.add("A");
        list.add("B");
        list.add("C");
        list.add("D");
        list.add("E");

        boolean found = list.contains("a");
        System.out.println("element a : " + (found ? "found" : "not found"));

        found = list.contains("B");
        System.out.println("element B : " + (found ? "found" : "not found"));

        int index = list.indexOf("c");
        System.out.println("element c : " + (index <= 0 ? "not found" : "found"));

        index = list.indexOf("C");
        System.out.println("element C : " + (index <= 0 ? "not found" : "found"));

        InsensitiveSearch test = new InsensitiveSearch(list);

        found = test.contains("a");
        System.out.println("element  : " + (found ? "found" : "not found"));

        found = test.contains("B");
        System.out.println("element B : " + (found ? "found" : "not found"));

        found = test.contains("c");
        System.out.println("element c : " + (found ? "found" : "not found"));

        found = test.contains("C");
        System.out.println("element C : " + (found ? "found" : "not found"));

    }
}

element a : not found
element B : found
element c : not found
element C : found
element a : found
element B : found
element c : found
element C : found

package com.roytuts.java.string;

public class StringStringBufferStringBuilder {

    public static void main(String[] args) {
        String s = "Hello, World!";
        System.out.println(s);
        s = "Welcome to Hello World!";
        System.out.println(s); // we lost reference to "Hello, World!"
        System.out.println();

        StringBuffer stringBuffer = new StringBuffer("Hello, World!");
        System.out.println(stringBuffer);
        System.out.println(stringBuffer.toString()); // convert to String object
        stringBuffer = new StringBuffer("Welcome to Hello World!");
        System.out.println(stringBuffer);
        System.out.println();

        StringBuilder stringBuilder = new StringBuilder("Hello, World!");
        System.out.println(stringBuilder);
        System.out.println(stringBuilder.toString()); // convert to String
                                                        // object
        stringBuilder = new StringBuilder("Welcome to Hello World!");
        System.out.println(stringBuilder);
    }

}

Hello, World!
Welcome to Hello World!

Hello, World!
Hello, World!
Welcome to Hello World!

Hello, World!
Hello, World!
Welcome to Hello World!

package com.roytuts.classexception;

public class ClassNotFoundExceptionTest {

    public static void main(String[] args) {
        try {
            Class.forName("ClassA"); // ClassA 没有定义
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

    java.lang.ClassNotFoundException: ClassA
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
at CoreJavaLearn.ClassNotFoundExceptionTest.main(ClassNotFoundExceptionTest.java:8)

import javax.jms.ConnectionFactory;

import org.apache.activemq.ActiveMQConnection;
import org.apache.activemq.ActiveMQConnectionFactory;

public class NoClassDefinitionFoundExceptionTest {

    public static void main(String[] args) {
        ConnectionFactory connectionFactory = new ActiveMQConnectionFactory(
                ActiveMQConnection.DEFAULT_BROKER_URL);
    }
}
``` 
 
+ 输出

```java
Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
ConnectionFactory cannot be resolved to a type
ActiveMQConnectionFactory cannot be resolved to a type
ActiveMQConnection cannot be resolved to a variable
at com.roytuts.classexception.NoClassDefinitionFoundExceptionTest.main(NoClassDefinitionFoundExceptionTest.java.11

// 使用final 关键字修饰函数即可
public void final funcname () {
}

public class abstract AbstractClass{
}

public abstract void AbstractMethod(){
}

System.currentTimeMills()   //可用于性能测试中