Debuger
Great questions
These questions will solve most bugs:
what method shows the symptom ? what lines of code produces that symptom ?
what is the state of the receiver object in that code ? what were the param values passed in ?
if it’s an exception, what does the exception error message say – null pointer? array access? Somethimes the exception name can be very informative.
调优方法
Eclipse debugger , 好用
println()
注释掉部分代码
Truths of Debugging
- 直觉很重要, 你可以测试你直觉的想法, 但当直觉与实际发生碰撞时, 实际发生获胜.
- 简单的代码也会引起重大的bug, 不要忽视那些简单的代码, 往往是它们出现问题.
- 注意你自己定义的变量, 程序中出现的bug, 往往是你定义的变量不是你想要的值.
- 如果你的程序1分钟前还可以正常执行, 但现在不行了, 那么你上次改动过什么? 注意: 如果你每写50行code就测试一下, 那么当程序出问题时, 你就 知道是哪50行出现的问题.
- 不要随便修改代码来追踪bug, 这样有可能带来新的bug.
- 如果你发现一些错误跟你一直追踪的错误没有关系, 那么先来将这些错误搞定吧, 没准这些错误与你一直追踪的bug是有关系的, 只是你还没想到.
Java 通用性(Generics)
- Using a generic class, like using ArrayList<String>
- Writing generic code with a simple <T> or <?> type parameter
- Writing generic code with a <T extends Foo> type parameter
Use Generic Class
ArrayList<String> strings = new ArrayList<String>();
strings.add("hi");
strings.add("there");
String s = strings.get(0);
循环使用
List<String> strings = ...
for (String s: strings) {
System.out.println(s);
}
例子:
public static void dempList() {
List<String> a = new ArrayList<String>();
a.add("Don't");
a.add("blame");
a.add("me");
for (String str: a) {
System.out.println(str);
}
Iterator<String> it = a.iterator();
while (it.hasNext()) {
String string = it.next();
System.out.println(String);
}
List<Integer> ints = new ArrayList<Integer>();
for (int i = 0; i<10; i++) {
ints.add(new Integer(i * i));
}
int sum = ints.get(0).intValue() + ints.get(1).intValue();
sum = ints.get(0) + ints.get(1);
// Generic Map Example Code
public static void demoMap() {
HashMap<Integer, String>map = new HashMap<Integer, String>();
map.put(new Integer(1), "one");
map.put(new Integer(2), "two");
map.put(3, "three"); // 自动包装
map.put(4, "four");
String s = nap.get(new Integer(3));
s = map.get(3) // 自动包装
HashMap<String, List<Integer>> counts = new HashMap<String, List<Integer>>();
List<Integer> evens = new ArrayList<Integer>();
evens.add(2);
evens.add(4);
evens.add(6);
counts.put("even", evens);
List<Integer> evens2 = counts.get("evens");
Define a Generic<T> Class/Method
you can define your own class as a generic class. the class definithion code is parameterized by a type, typically called<T>. This is more or less what ArrayList does. At the very start of the class, the parameter is added like this: public calss Foo<T>
这个 T 有以下限制: (T 的本质就好比是一个普通的类型)
- declare variables, parameters, and return types of the type T
- use = on T pointers
- call methods that work on all Objects, like .equals()
记住: where you see “T”, it is just replaced by “Object” to produce the code for runtime. So the ArrayList<String>code and the ArrayList<Integer>code … those two are actually just the ArrayList<Object>code at runtime.
例子:
public class Pair<T> {
private T a;
private T b;
private List<T> unused;
public Pair(T a, T b) {
this. a = a;
this.b = b;
}
public T getA() {
return a;
}
public T getB() {
return b;
}
public void swap() {
T temp = a;
a = b;
b = temp;
}
public boolean isSame() {
return a.equals(b);
}
public boolean contains(T elem) {
return (a.equals(elem) || b.equals(elem));
}
public static void main(String[] args) {
// integer 类型的可以
Pair<Integer> ipair = new Pair<Integer>(1, 2);
Integer a = ipair.getA();
int b = ipair.getB(); // 没包装
// String 类型的也可以
Pair<String> spair = new Pair<String>("hi", "there");
String s = spair.getA();
}
Generic <T> Method
与整个类都使用通用来说, 可以针对某个方法来使用通用, 语法: public <T> void foo(List<T> list)
例如:
public static <T> void removeAdjacent(Collection<T> coll) {
Iterator<T> it = coll.iterator();
T last = null;
while (it.hasNext()) {
T curr = it.next();
if (curr == las) it.remove();
last = curr;
}
}
<T> Method – use a <T> type on the method to identify what type of element is in the collection. The <T> goes just before the return type. T can be used to decalre variables, return types, etc. This is ok, but slightly heavyweight, since in this case we actually don’t care what type of thing is in there. This removes elements that are == to an adjacent element.
?/T with “extends” Generics
extends 可以限制 T, 例如 with a <T extends Number> – for any T value , we can assume it is a Number subclass, so .intValue()
例如:
public class PairNumber <T extends Number> {
private T a;
private T b;
public PairNumber( T a, T b) {
this.a = a;
this.b = b;
}
public int sum() {
return (a.intValue() + b.intValue());
}
?/T Extends Method
public static int sumAll(Collection<? extends Number> nums) {
int sum = 0;
for (Number num : nums) {
sum += num.intValue(0;
}
return sum;
}