java - 一旦我退出函数(Java)，对象的值为空

Question

import java.util.*;


public class Algorithm {

public static class Matrix{     
    private Double[][] x;
}

public static Scanner scan = new Scanner(System.in);
private static String str;

public static void read_data(Double[] radians, Double[] l) {
    l[0] = 0.0;
    int i;
    for (i = 0; i < 9; i++) {
        str = scan.next();  //passem la primera columna
        str = scan.next();  //agafem el valor del desplaçament
        str = str.substring(0, str.length()-1); //traiem la coma
        l[i+1] = Double.parseDouble(str);

        str = scan.next();  //passem la primera columna
        str = scan.next();  //agafem el valor del desplaçament
        if (i < 9) str = str.substring(0, str.length()-1);  //traiem la coma
        radians[i] = Math.toRadians(Double.parseDouble(str));
    }
    radians[i] = 0.0;
}

public static void print_Matrix(Double[][] M) {
    for (int k = 0; k < 4; k++) {
        System.out.print("\n");
        for (int j = 0; j < 4; j++) {
            System.out.print(String.valueOf(M[k][j]) + " , ");
        }
    }
}

public static void print_Jacobian(Double[][] M) {
    System.out.print("Jacobian Matrix =");
    for (int k = 0; k < 3; k++) {
        System.out.print("\n");
        for (int j = 0; j < 9; j++) {
            System.out.print(String.valueOf(M[k][j]) + " , ");
        }
    }
}

public static void init_Matrix(Double[][] M, int i, Double[] radians, Double[] l) {

    M[0][3] = l[i];
    M[0][0] = Math.cos(radians[i]);
    M[0][1] = -Math.sin(radians[i]);
    M[1][0] = Math.sin(radians[i]);
    M[1][1] = Math.cos(radians[i]);

    for (int k = 0; k < 4; k++) {
        for (int j = 0; j < 4; j++) {
            if (k == j && (M[k][j] == null)) M[k][j] = 1.0;
            else if(M[k][j] == null) M[k][j] = 0.0;
            //System.out.print(String.valueOf(M[k][j]) + "\n");
        }
    }
}

public static void init_Ultima_Matrix(Double[][] M, int i, Double[] l) {
    M[0][3] = l[i];
    for (int k = 0; k < 4; k++) {
        for (int j = 0; j < 4; j++) {
            if (k == j) M[k][j] = 1.0;
            else if(M[k][j] == null) M[k][j] = 0.0;
        }
    }
}

public static void init_Derivada(Double[][] M, int i, Double[] radians) {
    M[0][0] = -Math.sin(radians[i]);
    M[0][1] = -Math.cos(radians[i]);
    M[1][0] = Math.cos(radians[i]);
    M[1][1] = -Math.sin(radians[i]);

    for (int k = 0; k < 4; k++) {
        for (int j = 0; j < 4; j++) {
            if(M[k][j] == null) M[k][j] = 0.0;
        }
    }
}

public static void init_Ultima_Derivada(Double[][] M, int i) {
    for (int k = 0; k < 4; k++) {
        for (int j = 0; j < 4; j++) {
            M[k][j] = 0.0;
        }
    }
}

public static void fulfill_Ts(Matrix[] Ts, Double[] radians, Double[] l) {
    int i;
    for (i = 0; i < 9; i++) {
        Ts[i].x = new Double[4][4];
        init_Matrix(Ts[i].x, i, radians, l);
        //print_Matrix(Ts[i].x);
    }
    init_Ultima_Matrix(Ts[i].x, i, l);

}

public static void fulfill_Ds(Matrix[] Ds, Double[] radians) {
    int i;
    for (i = 0; i < 9; i++) {
        Ds[i].x = new Double[4][4];
        init_Derivada(Ds[i].x, i, radians);
    }
    init_Ultima_Derivada(Ds[i].x, i);
}

private static Double[][] product(Double[][] A, Double[][] B){  
    Double suma = 0.0;  
    Double result[][] = new Double[4][4];  
    for(int i = 0; i < 4; i++){  
        for(int j = 0; j < 4; j++){  
            suma = 0.0;  
            for(int k = 0; k < 4; k++){  
                suma += A[i][k] * B[k][j];  
            }  
            result[i][j] = suma;  
        }  
    }  
    return result;  
}

private static void calc_T(Matrix[] Ts, Double[][] T) {
    T = Ts[0].x;
    for (int j = 1; j < 10; j++) {
        T = product(T, Ts[j].x);
    }
}

private static void calc_Jacobian(Matrix[] Ts, Matrix[] Ds, int i, Double[][] jacobian) {
    Double[][] tmp;
    if (i == 0) tmp = Ds[0].x;
    else tmp = Ts[0].x;

    for (int j = 1; j < 10; j++) {
        if (j == i) tmp = product(tmp, Ds[j].x);
        else tmp = product(tmp, Ts[j].x);
    }
    jacobian[0][i] = tmp[0][3];
    jacobian[1][i] = tmp[1][3];
    jacobian[2][i] = tmp[0][0];
}



public static void main(String[] args) {
    Matrix[] Ts = new Matrix[10];
    Matrix[] Ds = new Matrix[10];

    for (int i = 0; i < 10; i++) {
        Ts[i] = new Matrix();
        Ts[i].x = new Double[4][4];
        Ds[i] = new Matrix(); 
        Ds[i].x = new Double[4][4];
    }

    Double[] radians = new Double[10];
    Double[] l = new Double[10];
    read_data(radians, l);
    fulfill_Ts(Ts, radians, l);
    fulfill_Ds(Ds, radians);
    Matrix T = new Matrix();
    T.x = new Double[4][4];
    System.out.print("\n Ts Matrix =");
    for (int q=0; q<10; ++q) print_Matrix(Ts[q].x);
    calc_T(Ts, T.x);
    System.out.print("\n T Matrix =");
    print_Matrix(T.x);
    Matrix jacobian = new Matrix();
    jacobian.x = new Double[3][9];

    for (int j=0; j<9; j++)
        calc_Jacobian(Ts, Ds, j, jacobian.x);
    print_Jacobian(jacobian.x);
    //La matriu Jacobiana hauria d'estar acabada
}
}

大家好 这是我的代码。问题是，在函数“calc_T(Matrix[] Ts, Double[][] T)”中，如果我在退出函数之前打印 T 的值，那就没问题了；但如果我在退出该功能后执行此操作，它会显示:

T 矩阵 = 空，空，空，空， 空，空，空，空， 空，空，空，空， 空，空，空，空

有人可以帮我吗？

非常感谢

Answer 1

这就是问题:

private static void calc_T(Matrix[] Ts, Double[][] T) {
    T = Ts[0].x;
    ...
}

Java 中的参数按值传递。所以基本上，您会忽略 T 的原始值，为其分配一个新值，然后不对它执行任何操作 - 这不会为调用者带来任何改变。你想要这样的东西:

private static Double[][] calc_T(Matrix[] Ts) {
    Double[] t = Ts[0].x;
    for (int j = 1; j < 10; j++) {
        t = product(t, Ts[j].x);
    }
    return t;
}

然后将其称为:

T.x = calc_T(Ts);

您还应该强烈考虑使用double[][]而不是Double[][]，除非您需要能够表示null，并开始遵循 Java 命名约定。