Summary explanation of some common problems in daily JAVA development

Preface

In an era of surging digital waves, program development is like a mysterious and magnificent magic castle, standing in the vast starry sky of technology. The characters in the code are like the twinkling stars, combined, intertwined and collided according to specific trajectories and rhythms, and are about to embark on a wonderful and creative journey full of infinite possibilities. When the blank document interface is like a deep universe waiting to be explored, programmers transform into fearless star pioneers, dancing lightly on the keyboard with their fingertips, preparing to use wisdom and logic to weave a program scroll that is enough to change the rules of the world's operation, and in the binary world of 0 and 1, they engrave the immortal mark of human innovation and breakthrough.

1. Syntax error-related issues

1.1 Missing or redundant semicolons

In Java, the statement ends with a semicolon. For example, it is common mistake to forget to add a semicolon after defining a variable.

Example:

int a = 10 // The missing semicolon here will cause a compilation error

Workaround: Check the end of the statement carefully to make sure each statement ends in a semicolon.

1.2 Bracket mismatch

This is common in control statements (such as if - else, for, while) and method definitions.

For example:

if (a &gt; 10) {
    ("a is greater than 10");
} else // A brace is missing here, which may lead to logical errors or compilation errors    ("a is less than or equal to 10");

Workaround: Check the brackets in the code to make sure the number of open and closed brackets matches and the range of the code blocks is as expected.

1.3 Variables are not defined or repeatedly defined

If you do not define it before using the variable, a compilation error will occur.

For example:

(b); // b is not defined, an error will occurint b = 20;

At the same time, it will also be an error to repeatedly define variables within the same scope.

int c = 30;
int c = 40; // Repeat the variable c, the compilation is not passed

Workaround: Define before using variables, and pay attention to the scope of the variables to avoid repeated definitions.

2. Data type related issues

2.1 Data type mismatch

Problems arise when trying to assign a value of one data type to another incompatible data type variable.

Example:

int num = 10.5; // An error occurs when trying to assign a value of type double to an int variable

Solution: Perform type conversion. If you are sure that the data accuracy will not be lost, you can use casting.

int num = (int)10.5; // This can convert the value of double type to int type, but the fractional part will be lost

2.2 Integer overflow and floating point accuracy issues

For integer types, an overflow occurs if the stored value is outside its range.

For example, for the byte type (range is - 128 to 127):

byte b = 127;
b = (byte)(b + 1); // The result will become - 128 because an overflow occurred

For floating-point numbers, accuracy issues may occur due to how they are stored inside the computer.

double d1 = 0.1;
double d2 = 0.2;
double sum = d1 + d2;
(sum == 0.3); // The result is false because of floating point accuracy issues

Solution: For integer overflow, consider using a larger data type or performing bounds checking. For floating point accuracy problems, you can use the BigDecimal class to perform high-precision numerical calculations.

III. Object-oriented programming related issues

3.1 NullPointerException

This is one of the most common exceptions in Java. When trying to access a method or property of an object and this object is null, a null pointer exception is thrown.

Example:

String str = null;
int length = (); // Because str is null, a null pointer exception will be thrown

Workaround: Before using the object, check whether the object is null.

if (str!= null) {
    int length = ();
}

3.2 Inheritance and polymorphism issues

Override methods incorrectly can cause problems. For example, the method signature (method name, parameter type and order, return type) is inconsistent.

Example of correct coverage method:

class Parent {
    public void print() {
        ("Parent print");
    }
}
class Child extends Parent {
    @Override
    public void print() {
        ("Child print");
    }
}

If the return type or parameter list is modified incorrectly, it is not a correct override.
Workaround: Check the method signature carefully to make sure that Java's override rules are followed. The super keyword can be used when you need to call the parent class method.

IV. Collection related issues

4.1 Concurrent ModificationException

This exception may occur when you modify the collection while iterating over a collection (such as ArrayList, HashSet, etc.).

Example:

ArrayList&lt;Integer&gt; list = new ArrayList&lt;&gt;();
(1);
(2);
for (Integer i : list) {
    if (i == 1) {
        (i); // Concurrent modification exception will be thrown    }
}

Workaround: You can use the iterator's remove method to safely delete elements, or use Java 8 streaming operations to process collections.

//Use an iteratorIterator&lt;Integer&gt; iterator = ();
while (()) {
    Integer i = ();
    if (i == 1) {
        ();
    }
}

4.2 Type mismatch and uninitialization issues (for generic collections)

Problems occur when adding elements of mismatched types to a generic collection.

Example:

ArrayList&lt;String&gt; stringList = new ArrayList&lt;&gt;();
(10); // An error occurs when trying to add an integer to the string list

In addition, if you forget to initialize the collection, a null pointer exception will also occur when using it.
Workaround: Make sure the type is correct when adding elements and initialize before using the collection.

5. Problems related to file operations and input and output

5.1 File does not exist or permission problems

Problems occur when trying to read or write a file that does not exist.

Example:

FileReader fileReader = new FileReader(""); // If the file does not exist, a FileNotFoundException will be thrown

Solution: Before operating the file, check whether the file exists. You can use the exists method of the File class. For permission issues, make sure the program has sufficient permissions to access the file.

File file = new File("");
if (()) {
    try {
        FileReader fileReader = new FileReader(file);
    } catch (FileNotFoundException e) {
        ();
    }
} else {
    ("The file does not exist");
}

5.2 The resource is not closed correctly

After performing file input and output operations or using a resource such as a database connection, if the resource is not closed correctly, it may cause resource leakage.

Example:

FileInputStream fis = new FileInputStream("");
// Forgot to close fis, it may cause the file handle and other resources to be unreleased

Workaround: Use the try - with - resources statement to automatically close the resource.

try (FileInputStream fis = new FileInputStream("")) {
    // File reading operation here} catch (FileNotFoundException e) {
    ();
} catch (IOException e) {
    ();
}

Summarize

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