初始化数组和类方法测试阵列时后NullPointerException异常 - Java的 [英] Java - NullPointerException after initializing array and when testing array with class method
问题描述
我一直在试图初始化数组，然后用一个类的方法测试其值。我已经初始化数组，并已经在构造函数中进行了测试成功，所以它看起来像阵列填充，但只要我尝试在主要的方法来测试这个它抛出一个空指针异常。
类PercolGrid {
双方诠释;
布尔[]网格; 公共PercolGrid（INT内侧）{
双方=内侧;
//格=新的布尔[] {假的，假的，假的，假的，假的，
//假的，假的，假的，假的，假};
布尔[] =电网新的布尔[两岸]
的for（int i = 0; I＆LT;两侧;我++）{
电网[I] = FALSE;
的System.out.println（设置+ I +到+电网[I]）;
}
的System.out.println（检查之外循环，第一方阵是：+电网[0]）;
}
公共布尔testSqr（int i）以{
的System.out.println（请求的指标是：+ I）;
回格[I]
} 公共静态无效的主要（字串[] args）{
PercolGrid firstGrid =新PercolGrid（10）;
的System.out.println（网格创建检查指标....！）;
的System.out.println（第一方阵是：+ firstGrid.testSqr（0））; // 空指针异常
的System.out.println（第一方阵是：+ firstGrid.grid [0]）; // 和这里
}
}
这几乎就像在构造函数中存在引用的数据，但随后在它之外不存在。当我注释掉为循环，它上面的布尔[] ... 行，并取消我的电网=新的布尔[] ... 行，这一切工作正常，但我想选边站的数量，当我实例化对象。
编辑 - 如果我注释掉，会出现此相同的错误行19（ firstGrid.testSqr（0）），并改为运行线20条（ firstGrid。电网[0] ）。
这是使用一维数组之前，我尝试用二维数组一样的东西的做法。我缺少什么？
我的输出是这样的：
0设置为false
...
9设置为false
检查外FOR循环，第一方阵的是：假的
网格创建！检查指标....
请求的指标为：0
显示java.lang.NullPointerException
在PercolGrid.testSqr（PercolGrid.java:19）
在PercolGrid.main（PercolGrid.java:25）
您的问题是这一行：
布尔[] =电网新的布尔[两岸]
这是初始化一个局部变量格，而不是字段中的实例。
将其更改为：
电网=新的布尔[两岸]
这初始化领域的实例。
通过将类型在前面您声明一个新的变量。当在方法declar一个变量的范围仅限于该方法。由于本地变量命名为与您的实例变量是隐藏的实例变量。
I've been trying to initialize an array and then test its values with a class method. I have initialized the array and have already tested it successfully within the constructor, so it looks like the array was populated, but as soon as I try to test this in the Main method it's throwing a NullPointer exception.
class PercolGrid {
int sides;
boolean[] grid;
public PercolGrid (int inSides) {
sides = inSides;
//grid = new boolean[] {false,false,false,false,false,
//false,false,false,false,false};
boolean[] grid = new boolean[sides];
for (int i = 0; i < sides; i++) {
grid[i] = false;
System.out.println("Setting " + i + " to " + grid[i]);
}
System.out.println("Checking outside FOR loop, first square is: " + grid[0]);
}
public boolean testSqr (int i) {
System.out.println("Requested index is: " + i);
return grid[i];
}
public static void main(String[] args){
PercolGrid firstGrid = new PercolGrid(10);
System.out.println("Grid created! Checking index ....");
System.out.println("First square is :" + firstGrid.testSqr(0)); // NullPointerException
System.out.println("First square is :" + firstGrid.grid[0]); // and here
}
}
It's almost like the referenced data exists within the constructor but then doesn't exist outside of it. When I comment out the for loop and the boolean[] .... line above it, and uncomment my grid = new boolean[] .... line, it all works ok, but I want to choose the number of sides when I instantiate the object.
EDIT - This same error occurs if I comment out line 19 (firstGrid.testSqr(0)) and instead run line 20 (firstGrid.grid[0]).
This is a practice using a 1D array before I try the same thing with a 2D array. What am I missing?
My output looks like this:
Setting 0 to false
...
Setting 9 to false
Checking outside FOR loop, first square is: false
Grid created! Checking index ....
Requested index is: 0
java.lang.NullPointerException
at PercolGrid.testSqr(PercolGrid.java:19)
at PercolGrid.main(PercolGrid.java:25)
Your problem is with this line:
boolean[] grid = new boolean[sides];
This is initializing a local variable grid, not the field in the instance.
Change it to:
grid = new boolean[sides];
This initializes the field in the instance.
By putting the type in front you are declaring a new variable. When you declar a variable in a method its scope is limited to that method. Since your local variable is named the same as your instance variable it "hides" the instance variable.
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