-
回答 3 已采纳 求教！ for(int i=0;i<array.length;i++)与
int length=array.length;for(int i=0;i<length;i++)哪个效率更高？
-
回答 2 已采纳 public class Array05 {
public static void main(String args[]){
int i,j,sum=0;
int num[][]={{30,35,26,32},{33,34,30,29}};
for(i=0;i<num.length;i++){
System.out.print("第"+(i+1)+"个人的成绩为：");
for(j=0;j<num[i].length;j++){
System.out.print(num[i][j]+" ");
sum+=num[i][j];
}
System.out.println();
}
System.out.print("总成绩是"+sum+"分！");
}
}
两次for循环中num.length和num[i].length分别是什么，是引用吗
-
回答 5 已采纳 ```
class SelectSort{
public static void Swap(int[] arr,int i,int j){
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
public static void PrintArray(int[] Array){
for(int i=0;iarr[j]){
Swap(arr,arr[i],arr[j]);
}
}
}
}
public static void main(String[] args){
int[] arr = {9,2,8,13,22,1};
SelectSort(arr);
PrintArray(arr);
}
}
```
/**
*@author 2015-09-26
*function:选择排序的练习和java中引用类型的用法
*问题：使用Swap()函数的时候，就会出现java.lang.ArrayIndexOutOfBoundsException问题
*目前还未解决，忧伤= =。
*/
-
回答 14 已采纳 public class Main {
public static double average(double[] array) {
double temp = 0;
double sum = 0;
for (int i = 0; i > result = ExcelUtil.readExcel(file);
ArrayList result2 = null;
double rand[] = new double[50];
double av;
java.util.Random r = new java.util.Random();
for (int j = 0; j < 10000; j++) {
for (int i = 0; i < 50; i++) {
try {
rand[i] = Double.valueOf(result.get(r.nextInt(62080)).get(0).toString()).doubleValue();
} catch (NumberFormatException e) {
e.printStackTrace();
}
System.out.println(rand[i]);
av = average(rand);
System.out.print("平均值为：" + av);
result2.add(av);
}
}
}
}
运行结果：
12.0
Exception in thread "main" 平均值为：0.24java.lang.NullPointerException
at Main.main(Main.java:35)
-
回答 5 已采纳 java语言程序设计第7章程序清单7-4
```
package Program7;
import Program6.Program6_10GetCharacter;
public class Program7_4 {
public static void main(String[] args) {
char[] chars = createArray();
System.out.println("The lowercase letters are: ");
displayArray(chars);
int[] counts = countLetters(chars);
System.out.println();
System.out.println("The occurrences of each letters are: ");
displayCounts(counts);
}
public static char[] createArray() {
char[] chars = new char[100];
for (int i = 0; i < chars.length; i++)
chars[i] = Program6_10GetCharacter.getRandomLowerCaseLetter();
return chars;
}
public static void displayArray(char[] chars) {
for (int i = 0; i < chars.length; i++) {
if((i + 1) % 20 == 0)
System.out.println(chars[i]);
else
System.out.print(chars[i] + " ");
}
}
public static int[] countLetters(char[] chars) {
int[] counts= new int[26];
for (int i = 0; i < chars.length; i++) {
counts[chars[i] - 'a']++;
}
return counts;
}
public static void displayCounts(int[] counts) {
for (int i = 0; i < counts.length; i++) {
if((i + 1) % 10 == 0)
System.out.println(counts[i] + " " + (char)(i + 'a'));
else
System.out.print(counts[i] + " " + (char)(i + 'a') + " ");
}
}
}
```
在 public static int[] countLetters(char[] chars) 方法里，用for (int i = 0; i < chars.length; i++) {counts[chars[i] - 'a']++;}可以实现对字符的计数，没能理解。哪位大神能说明一下么。书上解释没看懂。
-
回答 1 已采纳 #include
#include
using namespace std;
struct MyTreeNode
{
int col;//打印结果中的列
int row;//打印结果中的行
char data;
MyTreeNode *rightChild , *leftChild;
};
MyTreeNode* rebuild(string preorder,string inorder)//先序，中序
{
int i , len ;
MyTreeNode *root = new MyTreeNode() ;
root->data = preorder[0] ;
len = inorder.length() ;
for(i=0;ileftChild=rebuild(preorder.substr(1,i),inorder.substr(0,i));
}
else
{
root->leftChild = NULL ;
}
if(i!=len-1)
{//右子树
root->rightChild=rebuild(preorder.substr(i+1,len-1-i),inorder.substr(i+1,len-1-i));
}
else
{
root->rightChild = NULL ;
}
}
}
return root ;
};
void after(MyTreeNode *root)
{
if(root==NULL)return ;
{
if(root->leftChild!=NULL)
after(root->leftChild) ;
if(root->rightChild!=NULL)
after(root->rightChild) ;
cout data;
}
};
const int SPAN=4;//每一层的横向缩进值为4
unsigned char printResult[9][SPAN*4];//打印结果数组
int currRow=0;//第一轮行计数器
void printArray(unsigned char array[][SPAN*4],int length0,int length1)
{//打印结果数组
for(int i=0;irow=currRow;
currRow++;
root->col=level*SPAN;
printResult[root->row][root->col]=root->data;
preorderTraverse(root->leftChild,level+1);
preorderTraverse(root->rightChild,level+1);
}
//填充父节点与子节点的连线
void preorderForLine(MyTreeNode*root)
{
if(root==NULL)return;
int sCol=root->col;
int sRow=root->row;
if(root->leftChild!=NULL)
{//当前子树根节点到左子节点的连线
int eCol=root->leftChild->col;
int eRow=root->leftChild->row;
for(int i=sRow+1;irightChild!=NULL)
{//当前子树根节点到右子节点的连线
int eCol=root->rightChild->col;
int eRow=root->rightChild->row;
for(int i=sRow+1;ileftChild);
preorderForLine(root->rightChild);
}
//扫描结果数组将其右侧有横线的竖线替换为竖横线
void processAfterOne(unsigned char array[][SPAN*4],int length0,int length1)
{
for(int i=0;inul.");
string preorder , inorder ;
MyTreeNode *root = NULL ;
cout>preorder;
cout>inorder;
cout<<"post:"<<endl;
root = rebuild(preorder,inorder) ;
after(root) ;
cout<<endl;
cout<<"=====preorder traverse print binary tree====="<<endl;
cout<<"================"<<endl;
//先序遍历（计算每个节点打印位置）
preorderTraverse(root,0);
printArray(printResult,preorder.length(),SPAN*4);
cout<<"================="<<endl;
//先序遍历（填充父子节点到子节点的连线）
preorderForLine(root);
printArray(printResult,preorder.length(),SPAN*4);
cout<<"===================="<<endl;
//扫描结果数组将其中右侧有横线的竖线替换为竖横线
processAfterOne(printResult,preorder.length(),SPAN*4);
printArray(printResult,preorder.length(),SPAN*4);
cout<<"===================="<<endl;
//扫描结果数组将其中的下面没有竖线的竖横线替换为右拐竖线
processAfterTwo(printResult,preorder.length(),SPAN*4);
printArray(printResult,preorder.length(),SPAN*4);
cout<<"===================="<<endl;
return 0;
}
-
回答 5 已采纳 如代码，我发现程序在运行时会跳过cin.get，这是怎么回事啊？另外，关于将未知长度的字符串输入数组，还有什么好办法吗？
```
int main()
{
int time;
scanf("%d", &time);
int count=0;
while(count < time)
{
string str;
char temp;
int i;
while((temp=cin.get())!='\n')
{
str +=temp;
}
length =str.length();
char* preorder = new char[length];
char* inorder = new char[length];
for(i = 0;i<length;i++)
{
preorder[i]=str[i];
}
for(i=0;i<length;i++)
{
scanf("%c", inorder[i]);
}
sort(preorder, inorder, length);
count++;
delete preorder;
delete inorder;
}
return 0;
}
```
-
回答 3 已采纳 突然报SystemUI crash,查看log是数组越界：
04-23 05:23:58.484 2838 3037 E AndroidRuntime: FATAL EXCEPTION: Recents-TaskResourceLoader
04-23 05:23:58.484 2838 3037 E AndroidRuntime: Process: com.android.systemui, PID: 2838
04-23 05:23:58.484 2838 3037 E AndroidRuntime: java.lang.ArrayIndexOutOfBoundsException: src.length=23 srcPos=-14 dst.length=23 dstPos=-13 length=36
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at java.lang.System.arraycopy(Native Method)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at com.android.internal.util.GrowingArrayUtils.insert(GrowingArrayUtils.java:150)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at android.util.SparseArray.put(SparseArray.java:246)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at com.android.systemui.shared.recents.model.TaskKeyCache.put(TaskKeyCache.java:67)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at com.android.systemui.shared.recents.model.IconLoader.getIcon(IconLoader.java:77)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at com.android.systemui.shared.recents.model.BackgroundTaskLoader.processLoadQueueItem(BackgroundTaskLoader.java:140)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at com.android.systemui.shared.recents.model.BackgroundTaskLoader.run(BackgroundTaskLoader.java:109)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:873)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:99)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at android.os.Looper.loop(Looper.java:210)
04-23 05:23:58.484 2838 3037 E AndroidRuntime: at android.os.HandlerThread.run(HandlerThread.java:65)
查看代码是SparseArray.java的put方法插入值的时候i取了小于０，但是看不懂是怎么负值的：
public void put(int key, E value) {
// 二分查找,key在mKeys列表中对应的index
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
// 如果找到，则直接赋值
if (i >= 0) {
mValues[i] = value;
}
// 找不到
else {
// binarySearch方法中，找不到时，i取了其非，这里再次取非，则非非则正
i = ~i;
// 如果该位置的数据正好被删除,则赋值
if (i = mKeys.length) {
gc();
// Search again because indices may have changed.
i = ~ContainerHelpers.binarySearch(mKeys, mSize, key);
}
// 插入数据，增长mKeys与mValues列表
mKeys = GrowingArrayUtils.insert(mKeys, mSize, i, key);//这里i取了-14导致异常，不知道怎么回事．
mValues = GrowingArrayUtils.insert(mValues, mSize, i, value);
mSize++;
}
}
看了下SparseArray是线程不安全的，怀疑是不是多线程调用的时候被其他线程插入了数据，但是不知道怎么去排查．
-
回答 2 已采纳 
在主函数中,有语句
System.out.println("carl.equals(boss): " + carl.equals(boss));
也就是说 carl.equals(boss), 到manager函数中有
```
public boolean equals(Object otherObject)
{
if (!super.equals(otherObject)) return false;
Manager other = (Manager) otherObject;
// super.equals checked that this and other belong to the same class
return bonus == other.bonus;
}
```
这个super是调用父类中的方法,也就是说carl和boss先调用父类中的这个方法进行比较,这个是对的吗?
-
回答 4 已采纳 ```
public class EqualTest {
public static void main(String[] args) {
Employee alice1 = new Employee("Alice Adams", 75000, 1989, 12, 15);
Employee alice2 = alice1;
Employee alice3 = new Employee("Alice Adams", 75000, 1989, 12, 15);
Employee bob = new Employee("Bob Brandson", 50000, 1989, 10, 1);
System.out.println("alice1 == alice2: "+(alice1==alice2));
System.out.println("alice1 == alice3: "+(alice1==alice3));
System.out.println("alice1.equals(alice3): "+alice1.equals(alice3));
System.out.println("alice1.equals(bob): "+alice1.equals(bob));
System.out.println("bob.toString(): "+bob);
Manager car1 = new Manager("Carl Cracker", 80000, 1987, 12, 15);
Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
boss.setBonus(5000);
System.out.println("boss.toString(): "+boss);
System.out.println("carl.equals(boss): "+car1.equals(boss));
System.out.println("alice1.hashCode(): "+alice1.hashCode());
System.out.println("alice3.hashCode(): "+alice3.hashCode());
System.out.println("bob.hashCode(): "+bob.hashCode());
System.out.println("car1.hashCode(): "+car1.hashCode());
}
}
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Objects;
public class 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 getHireDay(){
return hireDay;
}
public void raiseSalary(double byPercent){
double raise = salary * byPercent / 100;
salary += raise;
}
public boolean equals(Object otherObject){
//这里获得一个对象参数，第一个if语句判断两个引用是否是同一个，如果是那么这两个对象肯定相等
if(this == otherObject)
return true;
//这里判断这个参数是否引用空值
if(otherObject == null)
return false;
//getClass()方法是得到对象的类，这里就是如果两个对象的类不一样，那么就不相等
if(getClass()!=otherObject.getClass())
return false;
//在以上判断完成，再将得到的参数对象强制转换为该对象，考虑到父类引用子类的对象的出现，然后再判断对象的属性是否相同
Employee other = (Employee)otherObject;
return Objects.equals(name, other.name)&&salary ==other.salary&&
Objects.equals(hireDay, other.hireDay);
}
//哈希散列，有自己的计算方法，根据字符串来得到一段数字
public int hashCode(){
return Objects.hash(name,salary,hireDay);
}
//toString()方法，可自动生成
@Override
public String toString() {
return getClass().getName()+ "[name=" + name + ", salary=" + salary + ", hireDay=" + hireDay + "]";
}
}
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 double getSalary(){
double baseSalary = super.getSalary();
return baseSalary + bonus;
}
public void setBonus(double b){
bonus = b;
}
public boolean equals(Object otherObject){
if(!super.equals(otherObject))
return false;
Manager other = (Manager)otherObject;
return bonus == other.bonus;
}
public int hashCode(){
return super.hashCode()+17*new Double(bonus).hashCode();
}
public String toString(){
return super.toString()+"[bonus=]"+bonus+"]";
}
}
```
这个程序里面有一部分，是public boolean equals（object otherObject）方法，
在这个方法的return语句里面莫名其妙的出现了一个Object.equals(a,b),我不明白这个是从哪里来的，求大神解释一下。
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