Java重要知识点（继承、多态、接口，异常，工具，测试）

重载、多态、接口

多态调用的顺序

当父类，和子类有Static时，先初始化Static，再初始化子类的Static，再初始化父类的其他成员变量－>父类构造方法－>子类其他成员变量－>子类的构造方法。

重写 hashcode equals compareto

Employee.java

import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Objects;

public class Employee implements Comparable<Employee> {
private String name;
private double salary;
private Date hireDay;

public Employee(String n, double s, int year, int month, int day) {
name = n;
salary = s;
GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
hireDay = calendar.getTime();
}

public String getName() {
return name;
}

public double getSalary() {
return salary;
}

public Date getHireDate() {
return hireDay;
}

public boolean equals(Object Otherobject) {
if (this == Otherobject)
return true;
if (Otherobject == null)
return false;
// if(! Otherobject instanceof Employee) return false;
if (getClass() != Otherobject.getClass())
return false;
Employee other = (Employee) Otherobject;
return Objects.equals(name, other.name) && salary == other.salary && Objects.equals(hireDay, other.hireDay);
}

public int hashCode() {
return Objects.hash(name, salary);
}

public String toString() {
return getClass().getName() + "name=" + name + ",salary=" + salary + ",hireDay=" + hireDay;
}

@Override
public int compareTo(Employee other) {
// if(getClass() != other.getClass()) throw new ClassCastException();
return Double.compare(salary, other.salary);
}
}

Manager.java

public class Manager extends Employee {

private double bonus;

public Manager(String n, double s, int year, int month, int day) {
super(n, s, year, month, day);
bonus = 0;
}

public void setBonus(double b) {
bonus = b;
}

@Override
public double getSalary() {
double baseSalary = super.getSalary();
return bonus + baseSalary;
}

@Override
public boolean equals(Object Otherobject) {
if (!super.equals(Otherobject))
return false;
Manager other = (Manager) Otherobject;
return bonus == other.bonus;
}

@Override
public int hashCode() {
return super.hashCode() + 17 * new Double(bonus).hashCode();
}

@Override
public String toString() {
return super.toString() + ",bonus=" + bonus;
}
}

异常

涉及return finally 以及异常抛出的问题

各个异常出现的情况

package exp;

import java.awt.Font;
import java.awt.FontFormatException;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import java.beans.Statement;
import java.io.EOFException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.sql.Connection;
import java.sql.DriverManager;
import java.util.Stack;

public class AllExcept {

public static void main(String[] args) throws Exception {
// 01 空指针异常 java.lang.NullPointerException
try {
int[] s = null;
int i = s.length;
} catch (Exception e) {
System.out.println("1、" + e.getClass().getName() + "异常");
}
// 02数组越界 java.lang.ArrayIndexOutOfBoundsException异常
try {
int[] s1 = new int[2];
s1[2] = 5;
} catch (Exception e) {
System.out.println("2、" + e.getClass().getName() + "异常");
}
// 03 错误类型的对象存储到一个对象数组时抛出的异常 java.lang.ArrayStoreException异常
try {
Object x[] = new String[3];
x[0] = new Integer(0);

} catch (Exception e) {
System.out.println("3、" + e.getClass().getName() + "异常");
}
// 04 除数为0异常 java.lang.ArithmeticException异常
try {
int a = 0;
int b = 10;
b /= a;
} catch (Exception e) {
System.out.println("4、" + e.getClass().getName() + "异常");
}
// 05 数据类型转换异常 java.lang.NumberFormatException异常
try {
String s2 = "19k";
int n = Integer.parseInt(s2);
} catch (Exception e) {
System.out.println("5、" + e.getClass().getName() + "异常");
}
// 06 超过字符串添加长度异常 java.lang.StringIndexOutOfBoundsException异常
try {
StringBuffer buffer = new StringBuffer();
buffer.charAt(5);
} catch (Exception e) {
System.out.println("6、" + e.getClass().getName() + "异常");
}
// 07 数组大小为负值异常。 java.lang.NegativeArraySizeException异常
try {
int i = -1;
int[] s = new int[i];
} catch (Exception e) {
System.out.println("7、" + e.getClass().getName() + "异常");
}
// 08 正则表达式异常 java.util.regex.PatternSyntaxException异常
try {
String a = "\\";// 错误的正则表达式
String a1 = "\\.";// 正确的正则表达式
String[] x5 = "abc.cde".split(a1);
x5 = "abc.cde".split(a);

} catch (Exception e) {
System.out.println("8、" + e.getClass().getName() + "异常");
}
// 09 空栈异常 java.util.EmptyStackException异常
try {
Stack s = new Stack();
s.pop();
} catch (Exception e) {
System.out.println("9、" + e.getClass().getName() + "异常");
}
// 10 找不到类异常 java.lang.ClassNotFoundException异常
try {
Object s = Class.forName("ClassName");
} catch (Exception e) {
System.out.println("10、" + e.getClass().getName() + "异常");
}
// 11非法参数异常
// 12 字体格式错误异常
try {
Font f = new Font("楷体", Font.BOLD, 7);
f.createFont(2, new File("1.txt"));
} catch (FontFormatException e) {
System.out.println("11、" + e.getClass().getName());
} catch (IllegalArgumentException e) {
System.out.println("12、" + e.getClass().getName() + "异常");
}
// 13不支持画图异常
try {
Image img = Toolkit.getDefaultToolkit().getImage("Images\1.jpg");
Graphics g = img.getGraphics();
g.drawRect(-1, -1, -1, -1);
} catch (Exception e) {
System.out.println("13、" + e.getClass().getName() + "异常");
}

// 14 BufferOverflowException
try {
int cap = 2;
ByteBuffer bf = ByteBuffer.allocate(cap);
for (int i = 0; i < cap; i++) {
bf.put((byte) i);
}
bf.put((byte) 10);
bf.getInt(9);
} catch (Exception e) {
System.out.println("14、" + e.getClass().getName() + "异常");
}
// 15文件找不到异常
// 16.当发生某种 I/O 异常时，抛出IOException
// 17.文件已结束异常EOFException
try {
File f = new File("E:\\1.txt");
byte[] buff = new byte[2];
FileInputStream file = new FileInputStream(f);
int bytes = file.read(buff, 0, 2);
System.out.println(new String(buff, 0, 0, bytes));
file.close();

} catch (FileNotFoundException e) {
System.out.println("15、" + e.getClass().getName() + "异常");
} catch (EOFException e) {
System.out.println("16、" + e.getClass().getName() + "异常");
} catch (IOException e) {
System.out.println("17、" + e.getClass().getName() + "异常");
}
// 18 ClassCastException异常
try {
Object s = new Integer(0);
System.out.println((String) s);
} catch (Exception e) {
System.out.println("18、" + e.getClass().getName() + "异常");
}
// 19 BufferUnderflowException异常
try {
ByteBuffer buf = ByteBuffer.allocate(50);
buf.flip();
byte[] byt = new byte[100];
buf.get(byt);

new RuntimeException();
} catch (Exception e) {
System.out.println("19、" + e.getClass().getName() + "异常");
}
// 20 java.sql.SQLException异常
try {

String sql = "select * from Table";// sql语句错误
Connection conn = DriverManager.getConnection("com.mysql.jdbc.Driver");

Statement smt = (Statement) conn.createStatement();
smt.execute();

} catch (Exception e) {
System.out.println("20、" + e.getClass().getName() + "异常");
}
}
}

工具

git svn 的 区别 和 常用命令

Maven依赖中的scope

测试

JUnit

Die.java

public class Die
{
private int faceValue;
private int numofsides;
public Die(){
faceValue = 0;
numofsides = 6;
}

public Die(int v){
numofsides = v;
}

public int numOfSides(){
return numofsides;
}

public int  topFace(){
return faceValue;
}

public void roll() {
faceValue = (int) (1 + Math.random() * numofsides );
}
}

TestDie.java

import org.junit.After;
import org.junit.Assert;
import org.junit.Test;
import org.junit.Before;

/**
* Unit test for simple App.
*/

public class TestDie {

private Die die1,die2;

@Before
public void setUp() {

die1 = new Die();
die2 = new Die(10);
die1.roll();
die2.roll();
}

@Test
public void testdie1numofsides() {
Assert.assertEquals(6, die1.numOfSides());
}

@Test
public void testdie1Face() {
Assert.assertTrue(die1.topFace() >= 1 && die1.topFace() <= 6);
}

@Test
public void testdie2numofsides() {
Assert.assertEquals(10, die2.numOfSides());
}

@Test
public void testdie2Face() {
Assert.assertTrue(die2.topFace() >= 1 && die2.topFace() <= 10);
}

@After
public void tearDown() {
}

}

实现 assertEquals aseertSame

assertEquals.java

当expected和actual都是String类型时，ComparisonFailure还会找出是前后相同的串，并用[Different String]标明那些不相同的字符串，也就是expectedToString和actualToString的格式将会变成：…${sameString}[${differentString}]${sameString}…。其中“…”只会在相同的字符串太长的情况下才会出现，这个长度标准目前（JUnit4.10）是20个字符。具体实现参考ComparisonFailure类，它继承自AssertionError，这里不再展开。

/**
* Asserts that two objects are equal. If they are not, an
* {@link AssertionError} is thrown with the given message. If
* <code>expected</code> and <code>actual</code> are <code>null</code>,
* they are considered equal.
*
* @param message
*            the identifying message for the {@link AssertionError} (<code>null</code>
*            okay)
* @param expected
*            expected value
* @param actual
*            actual value
*/
static public void assertEquals(String message, Object expected,
Object actual) {
if (expected == null && actual == null)
return;
if (expected != null && isEquals(expected, actual))
return;
else if (expected instanceof String && actual instanceof String) {
String cleanMessage= message == null ? "" : message;
throw new ComparisonFailure(cleanMessage, (String) expected,
(String) actual);
} else
failNotEquals(message, expected, actual);
}

private static boolean isEquals(Object expected, Object actual) {
return expected.equals(actual);
}

/**
* Asserts that two objects are equal. If they are not, an
* {@link AssertionError} without a message is thrown. If
* <code>expected</code> and <code>actual</code> are <code>null</code>,
* they are considered equal.
*
* @param expected
*            expected value
* @param actual
*            the value to check against <code>expected</code>
*/
static public void assertEquals(Object expected, Object actual) {
assertEquals(null, expected, actual);
}

aseertSame.java

/**
* Asserts that two objects refer to the same object. If they are not, an
* {@link AssertionError} is thrown with the given message.
*
* @param message
*            the identifying message for the {@link AssertionError} (<code>null</code>
*            okay)
* @param expected
*            the expected object
* @param actual
*            the object to compare to <code>expected</code>
*/
static public void assertSame(String message, Object expected, Object actual) {
if (expected == actual)
return;
failNotSame(message, expected, actual);
}

/**
* Asserts that two objects refer to the same object. If they are not the
* same, an {@link AssertionError} without a message is thrown.
*
* @param expected
*            the expected object
* @param actual
*            the object to compare to <code>expected</code>
*/
static public void assertSame(Object expected, Object actual) {
assertSame(null, expected, actual);
}

其他

hamcrest 可以用来增强 JUnit 中的 assert 功能
Cobertura is a free Java tool that calculates the percentage of code accessed by tests.
stub & mock. EasyMock has been the first dynamic Mock Object generator, relieving users of hand-writing Mock Objects, or generating code for them
DbUnit is a JUnit extension (also usable with Ant) targeted at database-driven projects that,among other things, puts your database into a known state between test runs

本文转自：http://blog.csdn.net/u011239443/article/details/53150159