java 知识点

1. 不清晰的知识点

package work.test;

import com.example.controller.UserController;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.*;
import java.math.BigDecimal;
import java.util.Calendar;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

/**
 * @author chenxinwei
 */
public class DoubleIntercept {
    private static Logger logger = LoggerFactory.getLogger(UserController.class);

    /**
     * 测试double的精度截取
     */
    @Test
    public void testDoubleIntercept() {
        int length = 4;//小数点后几位
        double value = 3.14d;
        BigDecimal bigDecimal = new BigDecimal(value);
        double result = bigDecimal.setScale(length, BigDecimal.ROUND_HALF_UP).doubleValue();
        logger.info("result:" + result);
    }

    /**
     * Map数据的存取
     */
    @Test
    public void testMap() {
        Map<String, String> hashMap = new HashMap<>();
        for (int i = 0; i < 10; i++) {
            hashMap.put(i + "", i * 10 + "");
        }
        //map能添加null
        hashMap.put(null, "test push null key");
        logger.info("hashMap size:" + hashMap.size());//11

        for (Map.Entry<String, String> entry : hashMap.entrySet()) {//输出的时候先输出 key为null的
            logger.info("key[" + entry.getKey() + "],value[" + entry.getValue() + "]");
        }
    }

    /**
     * 测试byte
     */
    @Test
    public void testByte() throws Exception {
        byte[] bytes = "1".getBytes();

        logger.info("bytes" + bytes.toString());
    }

    /**
     * 测试writeByte
     */
    @Test
    public void testWriteByte() throws Exception {
        //java新增二进制字面量
        int ivar100 = 0b11111111_11111111_11111111_10011100;//-100
        int ivar = 0b10011100;//-100
        logger.info("ivar[" + ivar100 + "]");
        OutputStream outputStream = new FileOutputStream("E:\\test.txt");
        outputStream.write(ivar);
        outputStream.flush();
    }

    /**
     * String compare 字符串截取 与比较
     */
    @Test
    public void StringCompare() {
        String dbfName = "ZL171204.dbf";
        String varyDate = "2017-12-04";
        String dbfVaryDate = varyDate.replace("-", "");
        logger.info("dbfVaryDate[" + dbfVaryDate + "]");
        logger.info("compare[" + dbfName.substring(2, 8) + "]");
        logger.info("dbfVaryDate[" + dbfVaryDate.substring(2) + "]");
        System.out.println(dbfName.substring(2, 8).equals(dbfVaryDate.substring(2)));
    }

    @Test
    public void testLong2String() {
        Long var = 0L;
        logger.info("var to String[" + var + "]");
    }

    @Test
    public void testTryCatch() {
        final String filePath = "E:\\log.txt";
        File file = new File(filePath);
        if (!file.exists()) {
            logger.info("文件不存在");
        }
        try (FileInputStream fileInputStream = new FileInputStream(file);
             InputStreamReader fileInputStreamReader = new InputStreamReader(fileInputStream);
             BufferedReader bufferedReader = new BufferedReader(fileInputStreamReader)) {

            String line = "";
            while ((line = bufferedReader.readLine()) != null) {
                if (line.equals("")) {
                    continue;
                }

                logger.info("line:" + line);
            }
        } catch (IOException e) {
            logger.error("获取文件流失败" + e);
        }
    }

    @Test
    public void dateAfter() {
        Calendar calendar = Calendar.getInstance();
        calendar.set(2017,11,13);
        Date varyDay = calendar.getTime();
        Date nextTradeDay = Calendar.getInstance().getTime();
        System.out.println("=============varyDay=================");
        System.out.println(varyDay.toString());
        System.out.println("=============nextTradeDay=================");
        System.out.println(nextTradeDay.toString());
        System.out.println(!varyDay.after(nextTradeDay));
    }

}

2. 反射

package work.test;

import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

/**
 * Created by IntelliJ IDEA.
 * File: TestRef.java
 * User: leizhimin
 * Date: 2008-1-28 14:48:44
 */
public class TestRef {

    public static void main(String args[]) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        Foo foo = new Foo("这个一个Foo对象！");
        Class clazz = foo.getClass();
        Method m1 = clazz.getDeclaredMethod("outInfo");
        Method m2 = clazz.getDeclaredMethod("setMsg", String.class);
        Method m3 = clazz.getDeclaredMethod("getMsg");
        m1.invoke(foo);
        m2.invoke(foo, "重新设置msg信息！");
        String msg = (String) m3.invoke(foo);
        System.out.println(msg);
    }
}

class Foo {
    private String msg;

    public Foo(String msg) {
        this.msg = msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    public String getMsg() {
        return msg;
    }

    public void outInfo() {
        System.out.println("这是测试Java反射的测试类");
    }
}